Chandler and Rachel do not seem to know each other. However, they had already met several years earlier on Thanksgiving 1987 at Monica's parent's house in The One with All the Thanksgivings (1998). Moreover, Rachel and Chandler made out when Monica and she visited Ross in college. Finally, Monica introduced Rachel to Chandler in the pilot as if they have never met before.
In this episode, Joey implies that he thinks that Chandler is gay, but in The One Where Nana Dies Twice (1994) he and Ross say that neither of them thought Chandler was gay when they met him.
When Ross and Phoebe are in the bar, Ross sets out shot glasses in a cluster. In the next shot, they are perfectly lined up. Shortly after, they are in a cluster again.
Rachel's friends saying that Rachel will be married "in a year" establishes that this takes place about a year before the first episode (as Rachel walks in during the pilot having left said wedding). In that episode Ross is also newly split up from Carol over her realizing she's a lesbian, as he is still depressed over his marriage ending and it turns out he got her pregnant near the end of their marriage. Yet in this episode - a year earlier - he declares that it turns out she's a lesbian and his marriage is over, making it a year-long depression and a year before Carol discovers she's pregnant, a pregnancy which still lasts all of the first season.
Chandler had already met Rachel when she and Monica were in high school. Ross and Chandler go home for Thanksgiving from college. Chandler should have remembered her when Monica introduced them in the bar.
Joey removes his clothes in Monica's apartment and is being covered by a lamp. For a moment you can see the top of a pair of shorts that he's wearing.
When Monica and Joey are flirting in the hallway and Monica is talking about the heat and humidity, someone can be seen behind her in her apartment through the crack of the door. But when Monica and Joey go into her apartment a moment later, no one's in there.
When Janice first asks the question who of the six of them have slept together, Ross and Rachel respond that none of them have. However those two are currently in a relationship and are in fact sleeping together. No one corrects them, not even Janice who should know that they are in a relationship.
In The One with the Boobies (1995), Chandler sees Rachel's breasts, leading to her trying to see him naked but seeing Joey instead. This, in turn, leads to Joey trying to see Rachel but seeing Monica instead. Finally, Monica tries to see Joey but ends up barging in on Joey Sr. Monica should be fine with leaving things be since she had a full body view of Joey on the day he moved in.