The issue with random applications failing to start (Firefox, Nautilus) or blowing up (panel, gvim) with BadName took me about 3 months to find (the bug was filed at the end of January). I'm not sure if my fix is any good, need to poke Ajax about it.
So... Wasted a lot of time, learned several mildly interesting things about the code and people involved.
The sad part is how much it takes to start moving around any modern codebase, and that's with the same language and toolchain. I remember times when no part of the system was off-limits, but these days... not so much. If anything breaks in OpenOffice, I'm not even going to try fixing it.