Ross tries to hide Chloe, the copy store girl he slept with, from Rachel on Friends.
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Friends Season 3 Episode 16 Quotes

No. I can't. You're a totally different person to me now. I used to think of you as somebody that would never, ever hurt me. Ever. God, and now I can't stop picturing you with her. I can't. It doesn't matter what you say or what you do, Ross. It's just changed everything. Forever.

Rachel

Ross: This can't be it.
Rachel: Then how come it is?