People press clear a lot when they mean to press enter. This is something that's been a problem since the day I started working at Old Navy. The only thing I can seem to link it to is that they expect the enter button to be where the clear button is, so they hit it without thinking, even though there's a HUGE enter button right beside it.
But it's not a big deal. When a customer says, "Oh, I hit clear instead of enter," I just say, "No problem, that just deleted the last number," and they'll enter the last number again and hit enter. Seriously, no problem.
This happened with a woman today. She typed in her four numbers, hit clear, and said, "Oh, oops, I hit clear."
I said, "No problem, just enter the last number again."
She laughs and says, "Geez, maybe if the clear button wasn't so much bigger than the enter button, it would be a little easier."
The enter button is literally three times the size of the clear button. Don't try to blame the card reader when you're obviously completely wrong.