Program shits itself randomly during execution, crashes, and doesn't tell you why. strace it. Oh, it turns out it's trying to execve() a binary that doesn't exist on the system, which wasn't documented as a dependency, so I didn't install it. Fixed.

Lots of little things like that. Why is this program acting slow at startup when it should be fast? Oh, because it's opening and timing out on a socket connection with an unusually long timeout. Et cetera...