"It's on special"
Seriously, all I do is put the barcode through the laser. If the bloody thing is on special, let me scan and if you don't see it, THEN complain so I can then rectify it. While I'm talking about specials, you can tell who failed basic maths because they complain they can't workout the multibuy deduction that appears.
"Are you open?"
This keeps cropping up. If I'm standing there, doing nothing, lights on, yes I'm open! Although, I must admit, I did once have this conversation not long ago:
5pm, peak hour
[Customer approaches, unloads]
Me: Sorry, I'm closed
Customer: OH FOR FUCKS
Me: Not!
[Both laugh]
I was absolutely stunned because it wasn't premeditated and I picked the worst time to do it and I didn't get a complaint. It was a complete stranger, not a regular customer. Though, I've decided to quit while I'm ahead.