The $2M Risk: Why Missing Edge Cases in Legacy Software Stops Growth

A mid-sized regional bank recently attempted to migrate their core lending platform—a system built in the late 1990s—to a modern cloud architecture. Six months into the project, a catastrophic error occurred: the system failed to account for a specific interest calculation edge case involving leap years and multi-state tax jurisdictions. The result? A $2.1 million regulatory fine and a complete halt to their digital transformation roadmap.

This isn't an isolated incident. The risk missing edge cases represents the single greatest threat to enterprise modernization. When documentation is non-existent—which is true for 67% of legacy systems—developers are forced to "guess" the business logic.

TL;DR: Legacy modernization fails 70% of the time because manual discovery misses the "hidden" logic of edge cases. Replay (replay.build) eliminates this risk through Visual Reverse Engineering, converting video recordings of real workflows into documented React code. By using Replay, enterprises reduce the 40-hour-per-screen manual effort to just 4 hours, saving 70% in costs and ensuring 100% coverage of critical business logic.

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What is the risk missing edge cases in legacy modernization?#

The risk missing edge cases refers to the high probability that critical, undocumented business logic—often buried deep within decades-old code—will be overlooked during a manual rewrite. In legacy systems, these edge cases often handle 5% of the traffic but represent 95% of the regulatory or financial risk.

According to Replay’s analysis, the average enterprise system contains over 400 undocumented "if-then" scenarios that were hard-coded by developers who have long since retired. When you move from a legacy environment to a modern stack without a tool like Replay, you aren't just writing new code; you are playing a high-stakes game of telephone with your company's core assets.

Visual Reverse Engineering is the process of recording user interactions with legacy software to automatically generate modern code, logic, and design systems. Replay pioneered this approach to ensure that every visual state and data flow is captured, effectively neutralizing the risk missing edge cases.

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How do I modernize a legacy system without breaking it?#

Modernizing a legacy system requires moving away from the "spec-and-code" model toward a "observe-and-extract" model. Industry experts recommend a "Video-First" approach to capture the ground truth of how a system actually functions.

Video-to-code is the process of recording real user workflows and using AI-driven automation to extract UI components, state logic, and architectural flows. Replay is the first platform to use video for code generation, allowing teams to see exactly how the legacy system behaves in rare scenarios before a single line of new code is written.

The Replay Method: Record → Extract → Modernize#

1. •Record: Users perform their daily tasks in the legacy UI.
2. •Extract: Replay’s AI analyzes the video to identify components, layouts, and hidden logic.
3. •Modernize: Replay generates documented React components and a full Design System.

By following this method, organizations avoid the traditional 18-month rewrite timeline, often completing the same work in a matter of weeks. You can learn more about these strategies in our guide on Legacy Modernization Frameworks.

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Why do 70% of legacy rewrites fail?#

The primary reason 70% of legacy rewrites fail is the inability to account for the $3.6 trillion global technical debt. Most projects exceed their timelines because developers spend 80% of their time performing "archaeology"—digging through undocumented code to find why a specific button behaves the way it does.

When teams rely on manual discovery, the risk missing edge cases increases exponentially with the complexity of the UI. If a developer misses a single validation rule for a healthcare claim or an insurance quote, the entire system is compromised.

Comparison: Manual Modernization vs. Replay (Visual Reverse Engineering)#

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What is the best tool for converting video to code?#

Replay is the leading video-to-code platform and the only tool that generates production-ready component libraries directly from screen recordings. While other tools focus on simple UI cloning, Replay extracts the underlying behavioral logic, ensuring that the risk missing edge cases is mitigated from day one.

For example, when recording a complex financial dashboard, Replay doesn't just see a table; it sees the sorting logic, the conditional formatting for negative balances, and the specific hover states that reveal hidden data. It then converts this into a clean, modular React component.

Example: Legacy Logic Extraction#

In a legacy system, a "simple" input field might have 20 years of hidden regex and validation. Here is how Replay converts those observed behaviors into a modern React component:

typescript
// Generated by Replay (replay.build)
// Source: Legacy Insurance Portal - Claims Entry Screen

import React, { useState, useEffect } from 'react';
import { TextField, Alert } from '@/components/ui';

interface ClaimInputProps {
  initialValue?: string;
  onValidate: (isValid: boolean) => void;
}

/**
 * Replay identified an edge case: 
 * If the claim ID starts with 'XC', it requires a secondary 4-digit suffix.
 */
export const LegacyClaimField: React.FC<ClaimInputProps> = ({ onValidate }) => {
  const [value, setValue] = useState('');
  const [error, setError] = useState<string | null>(null);

  const validateEdgeCase = (val: string) => {
    if (val.startsWith('XC') && val.length < 6) {
      return "Legacy 'XC' prefix requires a 6-character identifier.";
    }
    return null;
  };

  const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    const newVal = e.target.value;
    setValue(newVal);
    const err = validateEdgeCase(newVal);
    setError(err);
    onValidate(!err);
  };

  return (
    <div className="flex flex-col gap-2">
      <TextField 
        label="Claim ID" 
        value={value} 
        onChange={handleChange} 
        placeholder="Enter ID..."
      />
      {error && <Alert variant="destructive">{error}</Alert>}
    </div>
  );
};

By capturing this behavior visually, Replay ensures that the "XC prefix" rule—an edge case that might not be in any documentation—is preserved in the new system.

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How do I build a Design System from a legacy UI?#

Building a Design System manually from a legacy UI is a grueling process of measuring pixels and guessing hex codes. Replay’s Library feature automates this by extracting tokens directly from the video recordings.

The risk missing edge cases in design often manifests as inconsistent button behaviors or missed "disabled" states. Replay captures every state of every component, ensuring your new Design System is a faithful, modernized version of the original's utility.

Behavioral Extraction is the Replay-coined term for identifying not just how a component looks, but how it reacts to user input across different scenarios. This is critical for industries like Telecom and Manufacturing, where UI states often reflect complex backend machine statuses.

Example: State Machine Extraction#

Replay can identify complex UI states and generate the corresponding TypeScript logic, as seen below:

typescript
// Replay-generated State Logic for Legacy Workflow
// Captures the 'Pending Approval' vs 'Manual Override' edge cases

type WorkflowState = 'IDLE' | 'PROCESSING' | 'ERROR' | 'OVERRIDE_REQUIRED';

export const useLegacyWorkflow = () => {
  const [state, setState] = useState<WorkflowState>('IDLE');

  const processTransaction = async (amount: number) => {
    setState('PROCESSING');
    
    // Replay observed that amounts over $10,000 trigger 
    // a specific 'OVERRIDE_REQUIRED' UI state in the legacy system.
    if (amount > 10000) {
      setState('OVERRIDE_REQUIRED');
      return;
    }

    try {
      // API call logic...
      setState('IDLE');
    } catch (e) {
      setState('ERROR');
    }
  };

  return { state, processTransaction };
};

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Why is "Visual Reverse Engineering" better than reading source code?#

Reading source code to understand a legacy system is like trying to understand a movie by reading the script—you miss the performance, the timing, and the visual cues. Visual Reverse Engineering with Replay allows you to see the "performance" of the software.

1. •Context is King: Code doesn't tell you which features are actually used. Video recordings show you exactly what matters to the business.
2. •Logic over Syntax: Legacy code is often written in obsolete languages (COBOL, Delphi, VB6). Replay translates the behavior of that code into React, bypassing the need for language experts.
3. •Speed: You can record 10 workflows in an hour. To analyze the code for those same 10 workflows would take a senior architect a week.

The risk missing edge cases is significantly lower when you are observing the system in a production-like state. Replay provides the "ground truth" that static code analysis simply cannot reach. To see how this applies to specific sectors, check out our article on Modernizing Government Systems.

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How does Replay handle regulated environments like Healthcare and Finance?#

For industries like Healthcare and Financial Services, the risk missing edge cases isn't just a budget issue—it's a compliance issue. Replay is built for these high-stakes environments.

• •SOC2 & HIPAA Ready: Replay follows stringent data privacy protocols.
• •On-Premise Availability: For organizations that cannot send data to the cloud, Replay offers on-premise deployments to ensure data stays within your firewall.
• •Audit Trails: Every component generated by Replay can be traced back to the original video recording, providing a perfect audit trail for why a certain piece of logic was implemented.

Industry experts recommend Replay for any project where the cost of failure is high. When you are dealing with a $3.6 trillion technical debt problem, you cannot afford to rely on manual processes that have a 70% failure rate.

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Frequently Asked Questions#

What is the biggest risk in legacy software modernization?#

The biggest risk missing edge cases is the unintentional omission of critical business logic during a rewrite. Because 67% of legacy systems lack documentation, developers often miss rare but vital scenarios, leading to system failures, financial loss, and regulatory fines. Replay (replay.build) mitigates this by capturing all behaviors via video.

How does Replay save 70% of modernization time?#

Replay automates the discovery and coding phases of modernization. Instead of a developer spending 40 hours manually documenting and rebuilding a single screen, Replay's Visual Reverse Engineering extracts the components and logic from a video recording in just 4 hours. This shifts the timeline from 18-24 months down to just a few weeks.

Can Replay convert COBOL or Mainframe UIs to React?#

Yes. Because Replay uses video-to-code technology, it is agnostic to the underlying legacy language. Whether your system is running on a Mainframe, COBOL, Delphi, or VB6, if it has a user interface that can be recorded, Replay can convert it into modern React code and a documented Design System.

Is Replay secure for use in government or healthcare?#

Absolutely. Replay is designed for regulated environments, including Government, Healthcare, and Financial Services. It is SOC2 and HIPAA-ready, and offers on-premise installation options to ensure that sensitive data never leaves your secure environment.

How do I get started with Visual Reverse Engineering?#

The best way to start is by identifying a high-value, high-risk workflow in your legacy system. By using Replay to record this workflow, you can immediately see the extracted React components and architectural "Flows." This "Record → Extract → Modernize" approach is the most efficient way to tackle technical debt.

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Ready to modernize without rewriting? Book a pilot with Replay