First one I can think of is a cousin bringing me a friend’s car that the guy just installed a new head unit in and it had no sound. He even had the shop manual showing the pinout of the stereo. Problem was he looked at the diagram from the wrong direction, meaning sometimes they show connectors from the back or front but you have to pay attention to how you are viewing it. Somehow it powered up with 12v going to a speaker output, but it was way wrong. The head unit amp was trashed after this so after I rewired it I told my cousin his best bet was trying to get a replacement from the store. I used my DMM to confirm all of this and rewire properly, but it was brought to me after the damage was done.
One of my own personal mistakes I remember. I was testing an amp and swapped subs or rewired or something, all went well testing whatever amp it was, then changed back to my main amp and of course rewired the sub and put it back in the box (Kove Armageddon 15"). Well, sub was not making bass at all, but during my troubleshooting I noticed interior lights dimming real bad when bass played. As it turned out I accidentally wired one of the coils out of phase so it was just seeing an old MTX 1501D and not moving. Mind you I didn’t sit there and leave it cranked since there was clearly some issue, but would turn it up and check voltage and play with the RCA cables and such, I had presumed maybe a bad RCA cable for some reason at first. The sub worked once I corrected it, but it didn’t live too long after this mistake. Just a life lesson on not to rush things and double check.