Snow White and Prince Charming
A lot of us Barney/Robin shippers. And it was fairly obvious last episode that Barney knew Robin, Lily, and Ted were in the apartment.

Plus, one of the creators said there were a four episode arc starting a few episodes ago, which ends with Monday's episode.

From there, it was fairly easy to figure out Barney/Patrice wasn't real and she was in on the play.