in the episode when phoebe is replaced by Stephanie the guitarist, and phoebe is outside counting her money with Rachel and phoebe finds a condom while she did her smelly cat, and then the boy comes back and says can i have my condom back? isn't that phoebe brother later on? i think she would have noticed her brother don't you?
Giovanni Ribisi got the part of the condom guy. The producers liked him so much they invited him to audition for the part of Frank, and he got the part.
Just because the actor playing Phoebe's brother is the same man who played a very brief, small part in a much earlier episode does not mean that he was playing the part of her brother when he was getting his condom back. Obviously at this point, the writers hadn't yet established the part of Phoebe's brother, let alone the actor who would play that part. Therefore, this is not a slipup, just a small situation where the producers of Friends used the same actor twice, in different roles.
She'd never met her brother yet, so she wouldn't go "Oh look this person must be my brother" and I doubt anyone remembers every person who has ever said anything to them. It's not a slip-up, right enough the casting dirrctors didn't cast him as her brother yet, but he would have still been around and theoretically could have met and forgotten since it was such a brief meeting.
Yeah I don't think this qualifies as a slip up. Phoebe didn't know Frank Jr. at that point, and actually it doesn't even mean that Giovanni Ribisi was playing Frank at that time. So..not a slip up, but probably also something that not alot of people have caught! But trust me, I am a friends addict..I catch everything when it comes to this show! lol