I had spent about 3 months working on a feature for a CAD software in a company where I was an intern.
The day it was ready I commited everything to the main branch and asked a senior dev to check it.
It didn't work… we spent 30 minutes, tried almost eveything, but the software kept crashing (even if "it worked on my machine").
At that point he said "ok, we won't include this feature, it's ok"... even if I worked really hard for months to make it work, I felt so bad.

A few hours later I found out that for all this time I was trying it in debug mode, and a few types of errors were ignored, something which of course wasn't happening in release mode. Worst day in that company.

P.S.
The reason I wasn't testing it on release mode was the fact the solution was so big it took about 45 minutes to compile it (using IncrediBuild, compiling it using more than 10 machines at the same time), so I always used the debug mode to compile every small change in less than 2 minutes.