Corrected entry: In the flashback episode, Chandler and Monica are in the bar (which is soon to become Central Perk) and Rachel is in the same bar with her friends before she is due to marry Barry. When Chandler overhears Rachel - a complete stranger - say she wants one last fling, he tries to flirt with her, but she dismisses him. Monica then says "I know her" and talks to Rachel briefly with Chandler present. In fact, as revealed in later episodes, Chandler and Rachel knew each other when he was in college with Ross and Rachel was in high school with Monica - in fact they even made out at a college party.
Corrected entry: This episode takes place one year before the pilot (Rachel was supposed to get married in one year in the flashback and the pilot started on the day of her wedding). But in this episode, Ross finds out that Carol is a lesbian. However, not until the 2nd or 3rd episode does he find out she's pregnant. For this to make sense, either she had a really long pregnancy or else they continued sleeping together for a year after she came out (not likely).
Corrected entry: In 'The One With The Flashback' (episode 6, third series) Rachel appears to meet Chandler for the first time while celebrating her engagement to Barry, in the bar which was later to become the coffee house. In 'The One With The Thanksgivings' (episode 8, fifth series) it is apparent that she has already met Chandler twice at Ross and Monica's parents' house, once at Thanksgiving 1987 and once in Thanksgiving 1988.