Near the beginning of The Princess Bride, when Vizzini, Inigo, and Fezzik have kidnapped Buttercup and have her on their ship, they are surprised to find themselves pursued:
INIGO: You are sure nobody's following us?
VIZZINI: As I told you, it would be absolutely, totally, and in all other ways, inconceivable. No one in Guilder knows what we've done, and no one in Florin could've gotten here so fast.
For once Vizzini's logic appears to be correct: their abduction was well planned, and it does seem inconceivable that anyone would have discovered it already and be pursuing them. However, as Inigo points out immediately after the above quote, they do have a pursuer: the Man in Black.
How did he know that they'd kidnapped the princess in order to find them so quickly?