Spent three days wrestling with a UI rendering bug in our CMS editorial front end. Hampered by attritional relationship with CMS supplier, crappy documentation, and ropey code (ours and theirs). Googling further hampered by them shipping older, but unspecified versions of other libraries. Stumbled across solution by accident about 20 minutes ago. Fix required four additional lines of code.
I really, really hate situations like this. First, it's taken a ridiculously long time to sort out - bug was bounced to me from someone else and I think it was with another guy before that. Secondly, now that I have a fix I can't actually explain why it works, only that it does. That's rather rubbish and deeply unsatisfying.