For example, here is the definition of a "clearly erroneous" trade as deemed by Nasdaq: http://www.nasdaqtrader.com/trader/tradingservices/marketwatch/clearlyerroneousguide.pdf So if you get a print on a stock that is outside these ranges and is an abnormality, the trade could later be reviewed and busted, i.e., the trade is nullified.
there's also a couple other exceptions: someone places a huge size order and 'accidentally' does it. so the market goes down fast, in an orderly fashion, and they bitch and moan (they = large player) to the exchanges, and trades below a certain price get busted. OR... a random stock has a forged press release put out about it. stock tumbles hard. exchange busts trades below a certain price - so if you went long as it crashed, your 'long' is busted, but your 'sell' is not - meaning you're short a stock, and your profit is all gone. i've had variants of the second happen to me (when a stock shot up, i shorted, covered, then my short was busted). i've had the first happen a few times as well...