Every engineer eventually inherits a codebase with a history. At some point a ticket comes your way that sounds innocent. "Just refactor the billing module." Three story points. Four hours, tops.

Four hours in, you realise the billing module is held together by a comment from someone who left the company in 2011, and there are no tests.

Welcome to the cave.

The four-stage descent into legacy code

There is a reliable emotional arc to inheriting legacy code, and it plays out almost identically for every engineer, on every team, on every deadline.

Stage one: confidence. The ticket sounds simple. You have done refactors before. You roll up your sleeves, open the file, and feel briefly like an explorer about to make a clean discovery. You are wearing the metaphorical fedora.

Stage two: archaeology. Twenty minutes in, you are reading code that predates the company's current language. The variable names are cryptic. The class hierarchy is six levels deep. There is a function called doTheBillingThing(), and another called doTheBillingThingProperly(), and you cannot tell which one is being called by production. You git blame the file. The last commit is 2009. You feel the dust.

Stage three: dread. You find a comment. It is yellowed and weathered (metaphorically — your IDE renders it the same as everything else, but you can feel it). The comment says, in shaky block letters, "DO NOT TOUCH." There is no signature. There is no explanation. There is just the warning, sitting there, surrounded by the silence of nine commits in fifteen years. You should have brought backup.

Stage four: retreat. You back out of the file slowly, eyes locked on the screen as if the code might lunge at you if you turn around. You file a comment on the ticket. You suggest "perhaps we should scope this differently." You sleep poorly that night.

Why legacy code accumulates

Legacy code is not the fault of the people who wrote it. It is what happens when a working solution meets a decade of changing requirements, shifting personnel, and the slow accretion of workarounds. Every line was the right line at the time.

It just stopped being the right line three product pivots ago, and nobody had a spare sprint to rewrite it. The original author left. The colleague who understood half of it changed teams. The colleague who understood the other half changed companies. By the time the ticket reaches you, the code is a sealed tomb, and the only people who knew how it worked are gone.

The horror is not the old PHP, or the legacy Perl, or the abandoned Java 6. The horror is that if you touch it, seventeen other things break silently, and you will not find out until a support ticket comes in at 2am six weeks from now.

The "DO NOT TOUCH" comment is real

Every team has one. Sometimes it is literally that comment. Sometimes it is a slack message from a long-departed engineer that gets pasted into a wiki page and never updated. Sometimes it is a piece of folklore that gets passed down from senior to senior. ("Whatever you do, do not put a try-catch around the order-totalling function. We tried that in 2017. Do not ask why.")

The right move when you find a "DO NOT TOUCH" comment is not to assume the previous engineer was foolish or paranoid. It is to assume they had a reason, even if they could not write it down clearly, and to investigate before you act.

How to survive a refactor without losing your weekend

There is no escape from legacy code. There are only better ways to survive inside it:

Write a characterisation test before touching anything. Run the existing code through a battery of inputs and capture the outputs. The tests do not need to be "correct" - they just need to lock in current behaviour. After your refactor, the same inputs should produce the same outputs.
If the code is too tangled to test, that is the bug. Start by extracting just enough to make a test possible. The first refactor is "make the next refactor safer."
Keep the diff small. A 47-file refactor is a deployment-day catastrophe waiting to happen. Three-file diffs land cleanly.
Use Markdown Editor to draft the mental model of the system before you open the file. Writing it down forces honesty about what you actually understand.
Get sign-off on the scope before you start. "Refactor the billing module" is not a scope. "Replace the legacy invoice-totalling function with one that takes the new tax rules" is a scope.
Leave better comments than you found. Even if you cannot fix the cave, leave a torch burning for the next engineer. A comment that says "this looks weird because of X" is a gift to whoever inherits the code after you.

The cave will still be there tomorrow

The comic ends with Sam backing out and clutching the laptop. That is not failure. That is wisdom. Some refactors should not be done in one sprint, and sometimes the right move is to scope down, file a ticket for the bigger work, and ship the smaller fix first.

The warning scroll in the cave is real. Respect it. Someone wrote that for a reason. Find out the reason before you sand the warning off.