Forgetting to adjust the cutting height. A few years ago I had a customer who wanted his lawn kept short (2 inches). The customer I mowed for after his lawn wanted it longer (3 inches minimum).
I forgot to raise the deck before starting the second lawn and ended up scalping about 10 feet before realizing what had happened.
Luckily he and his wife were on vacation for two weeks, so it grew back without them noticing.
Since then, I always make it a point to double check.