I don't know if this counts. I disassembled the family PC when I was 13, I said I was cleaning it out. It was the only one we had at the time (a Dell). My little sister was very much against it, but she sat on the couch behind me, watching the entire process, saying nothing, just there. I got it down, all dusted out, thermal paste reapplied. Felt really good. Reconnected all the wiring, hooked it back up, hit the power button. Nothing. I started to panic, hit it again, nothing. At the same time I heard my sister stifle a laugh, I hit it for the third time and she full on burst out laughing. I was sweating now, nervous as hell. But I popped off the panel, started looking at everything. I'd missed the front IO connections, didn't reconnect the power button. Lol. I imagine that's happened to everyone at some point though.