I was installing a new, quieter dryer in a client's high-end house.
The power cord was unattached and had exposed connectors at one end.
Like an idiot, I pushed the plug into the 240-volt outlet to see if the prongs fit.
Well, the connectors at the other end were all touching one another and - WHAM! The loose ends shorted out and knocked me clear over.
This in turn set off the house alarm system, alerted the police and set every dog in the neighborhood barking.
It's bad enough to goof - it's far worse to have to explain your screwup to your client and your boss, a crowd of curious onlookers and the police.