Classic PC repair wisdom, errors and school-boy mistakes!...
1. Remembering to put bits back in that were taken out for troubleshooting
(having 1hr tangents because of what wasn't put back in or not reconnected during troubleshooting and eventually realising you were chasing another problem that didn't exist and was only happening because you left a piece out / disconnected! - a real time hole!)
2. Accepting time holes exist (as part of fixing anything)