Not a specific bug, but I always have that satisfactory feeling when I fix a bug inside a code like this:
1. Spaghetti af
2. Duplicated af
3. Heavy use of static fields instead of passing via callback when required
4. variable names like: textbox1 ..etc.
5. No comments

It is true that is a huge pain in the ass to deal with, but then I look at it as I was able to create something out of that mess, I mean all is mixed up nothing is clear, no clue where it starts and what caused it, but then I put the pieces together and end up with a solution of what I thought to be: It will never be solved when that mess is here.

Not an excuse for messed up code, I try fixing whenever possible, but hey, at least I did not run away and give up