I was newly hired to company. A customer came in yelling saying "there's a bug, this should do this but it's doing that..."
PM came to me and told me to "urgently fix this as this is an important customer".

So I started debugging for hours and asking around and all follow devs agreed that this is a bug. Then I found it!! And it was clear that it was not doing what the customer wanted.

I decided to look through this code history and found out that this part of the code wasn't changed for a year but the code commited before it did actually what the customer was expecting (whaaaa....)

Gathered the devs and the PM showing them what I found. They all looked at each other and then one said "ouuhhh right...yes it was doing this but we changed it to that..."

Turns out it's a feature not a bug, and everyone forgot about it.

FML