Quote from Strategery:
It's a legacy issue on the NYSE/AMEX. They have a shitty old system that fills odd lots on the last print instead of the bid for sells and the ask for buys. People take advantage of this in fast markets when the quotes are locked and enter hundreds of orders (BUY 99 MKT, BUY 99 MKT, etc) and get fills that they don't deserve. NYSE member firms know that the market surveillance people are so stupid that they can pretty much go after you for anything and win, they are probably sending everyone an email. What blows is that even if you send the order to an ECN, they are paranoid as well because they may have to route the order to the NYSE, hell they are probably flagging OTC stock odd lots too.
There are rumors of the rule going away, but this may not happen until the NYSE implements their CCG (common customer gateway) which probably wont happen till 2008.