It happens alot with the yucckky brokers (ETRADE AMERITRADE SCOTTRADE). Etrade you gotta call the corporate reorg and then they warn you that if you don't receive shares you'll go short. Ameritrade don't help at all. Scottrade sometimes forces through the orders if you complain.
Brokers clearing via Penson are pretty good, InteractiveBrokers is pretty good.
Just2trade and Successtrade, stay away from them boyyyyyyyyyyy.