That's happened maybe 4 or 5 times to me in the 200 trips or so (estimated, once a week for years) I've made since I started using the self-checkout every time. It's so reliable and fast for me (store: Jewel-Osco), I just don't have the problems others here are seeing. Usually while waiting in line I don't see anyone else have issues either.

Remove from basket, scan, put in bag. Remove from basket, scan, put in bag.

The scale is the entire bagging area, and it detects when things were removed or added without being scanned, as far as I know that's all it does here. No specific weights.