I can't stand the people that are cluelessly leaving their carts in the middle of the aisle as they select their brand of cereal.

Also hate those check-writers (contrary to what someone else posted, I find it's mostly the elderly, not females per se)

I do self check out when available, but I always bag my own. I can't stand when people just stand there starring as the cashier packs a full counter full of groceries.