I don't use stop orders, but it is my understanding that, unlike a limit order, a stop order becomes a market order when it is triggered. Thus, it is possible (and not unusual) to see stop orders executed lower or higher than the stated price because it is subject to the market AFTER the tiggering event. Now, unfortunately, there are phantom trades resulting from erroneous data getting into the trading systems, and such can trigger stop orders (read the disclaimers). These phantom trades, of course, never occurred, but sometimes show up in charts, etc., sometimes not. For an example, look at CIRT at http://stockcharts.com/ and you'll see a high of .007, but a look at 1 day chart of CIRT at http://bigcharts.marketwatch.com/ sees no such trade (obviously a misplaced decimal).