These things have become infamous here in the UK for "Unexpected Item in Bagging Area" and similar.
What would be really nice is RFID or similar instant scanning of everything in your basket (a local library has this for piles of books), but that may be a way off yet.
There are also products which do change weight. Electronic protection on clothes or wine bottles weight enough to confuse the scales sometimes. Condoms are packed in a plastic container which is taken off after purchase, but the weight is set for the condom pack only. I also get the error quite often because I'm thirsty/hungry and empty half of the bottle, or half a pack of brioches while shopping. (yes, that's my fault but I don't see any reasonable automated solution for it)
Then again, even with all those issues, I can still self checkout faster when doing small shopping alone (15+ items usually means some repacking so I think it's faster only when I'm there with my gf).
Seriously though, every now and again I've noticed some supermarkets upgrading to newer machines and invariably the newer machines are much less prone to these sorts of issues. In one supermarket where I sometimes shop, I had experienced the difference before and after the upgrade and it was significant.