You spend a few hours putting something back together. Two screws left over! You transplant a Macbook logic board into another unibody because this one is a bit tidier. You sell it. It's returned and you refund because two keys didn't work. (I know, I should have checked).
You get an iMac working. Can't find the RAM cover. An i7 CPU gets fitted into a motherboard...it POSTS! Yay, except that the POST freezes part wayu through and NOTHING you try appears to fix it.
These are the trials and errors of computer repair. They are frustrating, but also are the best way to learn stuff. They MAKE you check things that one last time (except when you get over confident and don't bother - then they bite you in the ****)
I quite enjoy it, really. Just as well I don't rely on it for my income...