The One With The Embryos - S4-E12
Other mistake: Ross makes up the questions for the quiz. There are two groups of questions, one for the girls and one for the boys. Each one has four categories; there are four questions in each category. From these 16 questions, Ross asks ten. Thus, there have to be six left after the quiz. But in one group, seven questions can be seen (there are seven cards left). (00:12:50)
The One With The Embryos - S4-E12
Other mistake: Phoebe takes a positive pregnancy test right after she gets implanted with the embryos. There's no test in the world that can tell that fast if someone is pregnant - even the best tests take at least a week after conception.
Suggested correction: Phoebe explains before she takes the first pregnancy test that her body's "always been a little faster than Western medicine". It's a deliberate joke.
A deliberate joke it may be, but we all know Phoebe believes a lot of silly things, and this is one of them. It really didn't make sense for her to get a positive result that fast.