[Radii]

Finally, I'm sure some of you noticed the very tall, red-headed man standing in the background of the early scenes at the bar. When Barney ran to go looking for the sex tape, he turned around and we could see for a moment that it was Conan O'Brien — who had won a charity auction for a background role on the show and specifically requested that he not be featured prominently. You either noticed him or you didn't.

From: Review: How I Met Your Mother - No Pressure: When you tell me that you love me - HitFix.com