Robin and Ted share a history. And it came out a bit on the How I Met Your Mother episode "Platonish."
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The friends look back at how Ted, Robin, and Barney navigated their complicated history on How I Met Your Mother. "Platonish" is the ninth episode of the show's ninth seaosn.
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How I Met Your Mother Season 9 Episode 9 Quotes

And the only survivor was me, because I was upstairs banging Ted's mom. What? Get that blonde girl's phone number? Challenge accepted!

Barney

Do you want to keep playing or do you want to win?

The Mother