Change Call?

[JLC]

In a game the other day, A1 went up for shot under basket. B1 went up for block. It appeared ref anticipated call (but I don't really know) becasue hand went up, but then he kind of half blew the whistle and had a look on his face like "Oh, that was really a good block and not a foul". But since he had blown the whistle he made the foul call on B1. My question is, has anyone blown their whistle and then wished they hadn't. And if so, do you go with the call anyway, knowing you missed that one? Do you have any other recourse?