Honestly, if I find a problem while I'm fixing another problem it does piss me off at first, but in the end I'm glad I know about it. Buying a part to "fix" something and then finding out in the process that the part you were going to replace is completely fine, yeah, that has happened to me and it is really annoying.

The thing I hate most is not knowing what is wrong with a car. At least you know what you have to do and can make plans to do it if you know exactly what's wrong. When you have a problem and can't figure out what's causing it, and have to go search forums for hours; you gather a list of everything that could possibly cause your problem and then you have to spend hours under the car checking every possible cause; finally it turns out that it's the one failure mode that nobody has ever heard of or ever experienced. Or worse, you exhaust every possibility and the car is still broken and you have to just start replacing shit until the car works again. Then two days later you see a post on a forum that describes your problem exactly and how to fix it in five minutes.

You can tell that troubleshooting is my least favorite part of the repair process.