There's so many ways a person could abuse self-service tills, it must happen all the time. I guess Tesco etc realise this, but its still worth it for them to have increased speed and reduced costs of employing someone to scan things.
On a small scale you could just put your red onions through as the less expensive white onions. Maybe pick up pink lady apples and put them through as golden delicious
I mean, whats going to happen? A security guard rifles through your bag and tells you off for not knowing the difference between apple types?
Surely if you did a big weekly shop no-one is going to notice if you fail to put one or two items through, instead just placing in a bag near the scales so they don't show up as excess weight - or even if you bought two of a particular item and only put one through - even if you somehow did get caught (which is surely very unlikely given staff cannot possibly watch every customer) you could simply say it was a mistake, that you are in a rush.
I know people who have cut-corners like this every time they've ever used a till, I've done it before when using coupons. In Tesco the machines ask you to insert your coupon when you've scanned your goods... but you can just not. it makes no difference.