In The Princess Bride, Vizzini was too smart for this own good. He was actually correct in deducing that he could drink from neither cup.

This is a revelation that seemed a little more obvious in the book. In the movie, it makes him look like he's being wishy washy and stalling, but he does in fact end up being right. And then when TDPR drinks, you can see Vizzini wait for him so that he knows it's okay to drink from his own cup. He did everything right except for accounting for the one thing he couldn't know, that TDPR was immune to the poison.

submitted by /u/megamoze
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source https://www.reddit.com/r/movies/comments/qi5b4g/in_the_princess_bride_vizzini_was_too_smart_for/