Lily tries to comfort Barney who is enraged at the idea of Ted and Robin sleeping together again. Lily tries to get Barney to admit he still has feelings for Robin.
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In this clip from the the upcoming episode of How I Met Your Mother, Robin and Ted are driving each other nuts as roommates. They think maybe have sex will solve the problem... much like Barney thinks it will solve world peace.
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In this scene from "Little Minnesota," Ted thinks his sister slept with Barney and he decides to confront her on it. A great reaction from both her and Barney.
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HIMYM Quotes

Whatever you do in this life, it's not legendary, unless your friends are there to see it.

Barney

You will be shocked kids, when you discover how easy it is in life to part ways with people forever. That's why, when you find someone you want to keep around, you do something about it.

Ted