Corrected entry: In "Friends" season 8, in "The One With Monica's Boots" A sister of Joey named Dina comes to talk to him about the fact that she is pregnant while she is still a teenager. You would think Joey would have said something like "Go ask our oldest sister Gina, she had Michael when she was 16" or something like that, but Gina isn't mentioned at all. One could say that this is a Friends-mistake, but we don't discover it until we meet Gina and Michael in "Joey", so.
Joey and the Dream Girl (1) - S1-E8
Corrected entry: When Michael and Joey are watching the tape with Michael playing baseball, Michael is holding the bat as he is right-handed. But when they are at the baseball-range, he is holding the bat as he is left-handed.
Correction: His character could be a switch hitter - Many people, myself included are able to bat left or right handed.
Correction: Joey was the one Dina was afraid to tell. Talking to Rachel was probably the best idea since both Dina and Rachel were in New York and Gina was in California. Dina was not looking to Joey for advice.