I just wasted 2 hours together with a colleague to trace a bug, through several modules, functions, data, etc to find out it was usage of the wrong information in the wrong place. The data used was never intended to be used this way.

I HATE SUCH SOFTWARE ARCHAELOGY.
Carefully uncovering layer over layer, getting one detail after another, from which you don't know if it's really necessary to trace the bug, until you lose the sight of the whole picture. Then when you're confused to the maximum, try to figure out what's important and what not and reassemble the puzzle until you can see where the road is heading.

At least we found the cause of the bug, so it wasn't useless. Now we have to waste more time to develop a solution (...preparing for next rant 🙇)