There's one Tesco in Oxford where they've done something genius - they have LOADS of self-checkouts, but to queue up to them you have to go through a little railed channel, that just so happens to have a turn that you can't get a trolley through. God bless them.
Trolley? -> manned checkout
Lots of booze and other age restricted stuff? - > manned checkout
Games/DVDs that need the case removing/disc fetching? -> manned checkout
500 coupons you've kept folded up in your wallet for 3 months and blatantly won't scan right, special offer reduced items with no special offer sticker, a card where the pin/chip doesn't work but you still want to use it, plus you're planning at the last second to 'remember' something and go back for it, leaving everyone waiting? -> firing squad
I also become mildly annoyed if you go in the 'express' checkout lane and someone has more than the maximum number of items and then seems surprised when everything's packed and sorted and they need to open a cavernous handbag and sort through it, rootle out a wallet and find their cash/card to pay for it. If you don't want to rush, don't go through the 'express' lane.