It varies between products as to what the default settlement period is. However, from my experience (which was in exchange traded options), companies would only

It varies between products as to what the default settlement period is. However, from my experience (which was in exchange traded options), companies would only have until the close of business to reverse (or bust) bad trades made on the exchange - any they'd have to meet certain criteria to do so, the bulk of which was, I think, that the trade had to have executed at a price which was more than a certain percent away from the current market price.