The photo shown of Robin on Letterman is not photoshopped - this is from Cobie Smulders' actual appearance on the show in 2011.
At the end of this episode, Ted walks into the bar in a green dress and a pregnant Lily and Barney hugging. This is the explained story that Ted couldn't remember from season 6, episode 11, The Mermaid Theory.
In 'Now We're Even' Lily has a sex dream featuring Papa Smurf. Neil Patrick Harris starred in The Smurfs (2011) movie alongside 'How I Met Your Mother' guest star Katy Perry.
Barney misuses the word "epiphany" in this episode. In Episode 8 of Season 1, The Duel, he uses the word correctly.