Other mistake: In 'The One with the Ick Factor' (Season One) Monica states that she is 26 years old. This episode takes place in 1994/95. However there are numerous other episodes such as 'The One With All The Thanksgivings' that show Monica and Rachel (who are the same age and in the same class) still being in High School in 1988. In 'The One Where The Stripper Cries' the two have to sneak into a college party in 1987 as they are still high school age. They should be at least 19 or 20 in 1987/88.
Other mistake: In almost every season when the friends are in the coffee shop, it shows a front view of Central Perk; and a lot of times, in different episodes, the same (unaging) lady with blonde curly hair is walking past the shop. There are also a couple of different episodes where two babies with red hair are being carried past the coffee shop.
Other mistake: In his apartment from later episodes, Ross' kitchen door is on the same wall as the front door, and it is a long and narrow kitchen, but Ross' apartment is between two others. The floor plan doesn't make sense unless the kitchen is only a foot wide, otherwise it would jut off into the hallway.