What's the proper action?

[PantsOnFire]

These kind of things are governed by common sense for the general rule and poker sense for the specifics.

Obviously, BB has the right to fold, call or raise. And he can't do that seeing the flop cards. Thus, the flop cards are no good, BB now acts and we get new flop cards.

So how do we get new flop cards? Well we put the old ones back into the stub, shuffle and get three brand new ones. (Note: Stub is the unused portion of the deck.)

That was an easy question. A hard one would be what if UTG now moved all-in and somebody called before BB could speak up?