Other than overnight gaps or trading halts, are there any circumstances in which a resting limit order (one that is not filled immediately upon placement) can be filled at a better price than the limit price? If so, what are those circumstances?
A sudden downdraft or updraft where bids or asks disappear can cause this. Many years ago I had an order open for several weeks to buy LU at $20. One day there was a crash in the middle of the session and I got filled at 19 3/4. (I don't think it ever got above 20 again )