It is possible depending on what data feed you are looking at. On NYSE issues (NYSE only trading), there are trades that are done on a third market outside of the NYSE (think ECN like ARCA). In a name such as IBM trades start right at 9:30am EST even before the stock is open on the NYSE, The consolidated tape shows all trades, but on the open of the market on the NYSE the stock often opens after 9:30AM EST, when the opening cross is posted. It is entirely possible that many third market prints happen BEFORE this official NYSE open, and it is possible that those are above this open, which might be the high for the day NYSE only. The key here is there is the official NYSE open, and then the first print open, which are often very different prices. However, the high of the day would reflect the trades that happen prior to the NYSE open, but if you were trading with the specialist only, its possible those prints are above (or below) the high/low traded for the day on the NYSE only.