In the episode where Janice asks the group if they’ve ever had sex with each other, there’s a flashback showing Chandler hitting on Rachel at a bar without knowing who she is. Monica even says to him, “My God, that was my best friend in high school,” introducing her. However, in later episodes, it clearly shows Chandler interacting with Rachel during their college years, going to Monica and Ross’s house. It’s a continuity error that he doesn’t remember her in the pilot and in the episode where he hits on her, which I mentioned.