Fat fingers and broken algorithms happen. Venues publish break thresholds — for example a percentage away from the reference price, tighter for liquid names — and both the filing window and the review are measured in minutes, not days.
For a trader this cuts both ways. A beautiful fill 40% away from the market may simply be taken away hours later, and any hedge placed against it is now naked.
Example: an algorithm sells 5,000 shares of a $60 stock down to $34 in one sweep. You buy 1,000 at 34.00, then sell them back at 58.00 for a $24,000 gain. The exchange busts the erroneous prints below $54. Your buy is cancelled, your sale stands, and you are now short 1,000 shares at 58.00 with no offsetting position.
Related: limit-up-limit-down, fast-market, kill-switch, trading-halt