David Letterman & Neil Patrick Harris' Awkward Green Room Encounter (VIDEO)

Neil Patrick Harris stopped by Letterman on Wednesday night to discuss -- among other things -- his adorable toddlers attempting to play the slots in Vegas.

But in addition to his interview segment, Harris was part of a rare Letterman cold open, wherein the actor paid the host a sincere compliment before the show in the green room.

Clearly, Letterman just isn't great at taking compliments.