When do you finally step in?

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One cannot use rule 10 and place the runners where he thinks the runners would have been in this situation. Once "foul ball" is called, the play is dead. To allow the umps to reverse the call to fair and place the runners where they think they should be, the umps would have to assume everyone would have been safe. The play is dead, and you go to the next pitch.
Reversing the foul ball to fair and placing runners is worse than the original call of miscalling a foul ball.