Funny this came up, here is a response from Joe Brinkman on the World Umpires Ask the Umpire Webpage.
If the umpires forget to call/signal for an infield fly, and the ball is not caught, what should be done? Likewise, what happens if the umpires call/signal an infield fly when by rule (i.e., there are two out) the infield fly does not apply, and the ball is not caught? (Dick)
If there was supposed to be an infield fly, but you let it get away from you without calling it, you have to let the play go and see what happens. If the dust clears and the defense got a double play that the infield fly rule was meant to prevent, you have to step in and make things right, which would mean calling the batter out and sending runners back, nullifying one of the outs. If an infield fly was called but could not have been an infield fly by rule, the players must ignore the call and the results of the play would stand. The players are responsible to know the rule and number of outs, just as you are.
http://worldumpires.com/askUmp.htm