My personal favorites are the ones that scan everything, then start bagging everything, then start looking for their card in their handbag, shoulder bag, backpack, pockets etc.
My biggest complaint about self checkout at Walmart (specifically) is that I still have to wait in line! There's 20-ish self checkout machines of which 15-20 will be working and like fucking forty regular checkout lanes with two cashiers working. So of course there's going to be a long ass line for the self checkout!
It's lane upon lane of wasted space. If you're only ever going to have 3-4 people working then you should only have 3-4 non-self checkout lanes!