Earlier today at work I lost nearly 2 hours scratching my head wondering why I couldn't get an element's width right using standard padding / float tricks. I finally found the problem, a forgotten min-width call was being inherited from the ruleset meant for tablets. Easy enough to fix (min-width: 0; to override) but the min-width call wasn't being called out on Firebug which is why I didn't spot it quicker. :\
What made it especially fun was the fact I built out a testcase version of the sheet and couldn't replicate the bug (because I didn't test the other screen modes).
So, anyone want to share their own multi hour layout problems that, once found, led to a moment of "Doh!" ?