I saw a lovely example of this a few months agp. We had a list of ids representing completely different business cases, and code like this:

  // Because of [sane business reason] we need to remove all 89's
  something.removeall(75);

I found this because people in case 75 were logging a never ending stream of tickets for months that the program was doing weird things. No shit, sherlock. Talk to business, everyone agrees 75 makes no sense at all and it really really should be 89. So before we change this back to sanity, let's check the git history:

  Date: 3 years ago
  Message: Urgent fix for major downtime.  (No detail of course)
  Commit by: someone who left a little bit later 
  Change: 1 line patch, changes 89 to 75

Asking around, nobody technical knows anything anymore, the whole team got laid off around that same time 3 years ago and the new team basically rediscovered everything from scratch. Some business people do remember it, it was Very Bad, they don't know exactly what it was or how it got fixed, but anything that might cause it to happen again is strictly forbidden.

What would you do? I got reorganized to another team, lucky me.