The Erratic Amigo
No disrespect to people who need motorized carts to do their shopping, but maybe a training course should be required to operate one. It would prevent dozens of displays from being destroyed, and dozens of run-over feet courtesy of old people who ramp these things up to full speed, then cruise around like they're playing Mario Kart after huffing paint.
The Cell Phone Wanderer
Because there's no better time to answer a random call than right this minute, this species of jackass will immediately abandon his cart in the middle of an aisle, then spend 15 minutes wandering around the store, picking up random crap, looking at it, and putting it back as he talks about everything from socks to weekend plans. It's best to relocate his cart for him, then track him as he gets even more confused and frustrated.