There are two things I find really annoying at the supermarket:
1)People who park their cart dead-center in the middle of an aisle so you can't get around them.
2)People who come up behind you on the self-checkout line and start their order before you're finished bagging your own. Thank you, but I'd rather not have my bread smashed by your 12-pack of Snapple!
Yep, someone needs to make an official 'Grocery Etiquette' guide!