Quote from Copernicus:
of course they wont let, but when things go bad they go bad, when things gap risk management can only watch, same thing when someone jumps in with a basket when things are moving, by the time they cover your positions, your 5k or whatever is long gone. Airport trade on a supposed mistake 5 seconds before the close will do the trick too. Supposedly you are long/short 5k of stock with next days earnings, 5 seconds to close you mean to cover, but "somehow" you put in wrong order to double your position.
You're making it sound like this happens all the time when reality is that this is the exception more than it is the rule.