Andy,
Back in the 70's I did a valve job on my aunt's car. I was just torquing down the head when a (formally licensed) Master heavy-duty mechanic buddy stopped by. We backed the beastie out of the garage and finished up connecting things. Another buddy who was an NHRA track mechanic showed up as we were refilling the fluids. My neighbor's 10 year old son wandered by to watch.
We were set to go and started cranking. RRR, rrr, RRR, rrr, nothing. "Oh, <bleep> I must have installed the cam 180° out!" 20 minutes of flashing wrenches to check that everything was good in that department. RRR, rrr, RRR, nothing. We tried this, we tried that, nothing.
My neighbor's kid picks up the rotor that was sitting in the wheel well cover and asks, "Doesn't this go somewhere?"