Gaps happen where the market is closed or paused: overnight, over weekends, across earnings, around scheduled data, and through trading-halts. No stop placement survives a gap, because there is no liquidity at the skipped prices.
The only genuine defences are structural: smaller size over event windows, defined-risk options positions, or simply not holding through the event.
Example: a trader is long 2,000 shares at 45.00, stop at 43.50, risking $3,000. The company pre-announces after the close and the halt-auction reopens at 31.00. The realised loss is $28,000 — more than nine times the planned risk, with the stop working perfectly the whole time.
Related: stop-order-slippage, halt-auction, extended-hours, opening-auction