Another common bug: bad troubleshooters don't verify their hypotheses. They get attached to their first idea of what might be going wrong, and start tweaking things to alter the behavior of whatever they think is at fault --- and after each such tweak fails, it's on to the next, without a pause to consider that the problem might be elsewhere. Time wasted like this can range from hours to weeks.
Where this gets really annoying is when you present them with direct evidence that the problem is elsewhere, and you get back an emotionally driven argument for ignoring it...
Yes, when I debug I first verify all the obvious things with quick sanity checks, and then do a binary search down to the more detailed possibilities. Just yesterday I helped my friend find an inscrutable bug. He checked the inputs (he expected "myString" and indeed got "myString") so he delved deep into the system fruitlessly for hours. When I helped, I quickly uncovered that "myString" != "myString": his input had a carriage return prepended to it!
Where this gets really annoying is when you present them with direct evidence that the problem is elsewhere, and you get back an emotionally driven argument for ignoring it...