It's always a great feeling when a solution you made for a pet project last night turns out to be exactly what you need for issues that arose today.

The build broke right before code review with the Lead. It didn't just break in one place, oh no, that would be too simple. It broke everywhere, right down to the core mechanic. I spent the next 3 hours trying to find out why it broke, checking everything involved in this part of the system. It was a freaking Initialization call placed inside a conditional statement instead of outside.