Factual error: The symptoms (spots, fever, etc) of chicken pox only develop 2 - 3 weeks after exposure to the virus. Therefore, when Ross knew that Ben had chicken pox, Phoebe shouldn't have developed the condition until at least two weeks later, and Ryan certainly shouldn't have developed the disease until the end of his stay (as Phoebe was his only contact with the virus).
Factual error: In the one where Joey and Chandler lose baby Ben on a transit city bus, they later have to pick him up at the transit city headquarters. The man working there assumes that one of them is the father. There is no way that a city official would just give away a baby without checking for ID or a birth certificate or proof of fathership of the child.