While fixing the problem other usually reliable things mysteriously go wrong - like tank traps in your path. The USB CD or floppy reader doesn't want to work. The login to download a tool no longer works - so you have to re-register.. Then you find your broadband has decided to go TITSUP halfway through the download.
You can always tell when you are starting to win - the obstacles become less and less subtle. Like the power-on button breaking or even a power failure.
Finally you discover that the reason the obvious fix didn't work - is that little change you made during the first stage of diagnostics. Even worse if it is one of the minor distractions of "That looks wrong - I'll tidy that while I'm here. So simple there's no need to test it".