10 More Annoying Mistakes You Never Noticed In Friends
6. Rachel's Father Smokes
In Season 2 Rachel has a double birthday party in "The One With The Two Parties" since her parents can't be in the same room. Here we see her father quite obviously smoking cigarettes. It's emphasized when Rachel's mother catches Ross holding her ex-husband's things. Ross pretends like they're his, putting on the glasses and popping a cigarette in his mouth so that she won't be aware that Rachel's father is there. Rachel's mother then tells Rachel that Ross is so similar to her father, indicating that he must have smoked while they were still married too.
In Season 5 in "The One Where Rachel Smokes" she claims that she doesn't smoke because her father told her horror stories about it (as a doctor he should know). So either he is a hypocrite who is warning her of his vice, or the writers just forget that he was seen smoking earlier.