optionsXpress wouldn't fill the .50 order because.
Post# of 11038
In general, a limit above the ask would fill. It's called a marketable limit order. However this is more than 50x the ask. Order like that are often rejected since they appear to be an error. For example, and order to buy JCP at 90.55 would likely be rejected since it's 10x the current market for that security.
It's a protective measure to help prevent unfavorable fills.