Phoebe's birthday also changes during the season. Early in the series she tells her brother that her birthday is in February but late in the series when they're all late for her birthday dinner, they are celebrating it on Halloween, which is creepy to Joey.
When Monica approaches Chandler when he comes home, the camera loses focus on Monica briefly.
When Ross and Rachel get locked out of the apartment you can see down the corridor which splits left and right at the end. This leaves no room for Ross' kitchen which should be before the end of the corridor.
Ross says that Rachel has not cooked since 1996. However in "The One Where Ross Got High" (1999), Rachel did prepare a trifle. Aside from preparing custard, this dessert normally does not require much technical cooking (with heat). But Rachel misinterpreted the recipe and included a layer of beef sautéed with peas.
Claudia states that it is legal to smoke in Oklahoma offices with "15 people or less". It should be "fewer".