Nearly at the middle of the episode, while Joey, Alex and Gina are in Joey's apartment discussing Alex's issue, we see Alex sitting with a yellow shirt with a folded sleeve, when the camera position changes the sleeve is not folded, then on the third view when Alex is standing up the sleeve is again folded.
Joey says at the beginning of the episode that he has dated women from all over the world except for Iceland, and is pleased to discover in the end that Sara is part Icelandic. But when he takes a map right after this discovery with red pins for each country from which he's ever dated a woman, we can see that Iceland already has a red pin. (However, this could actually be deliberately made by the film-makers to show how dumb Joey is. He has trouble multiplying 7 and 9 - he could've put the pin on Iceland thinking it's Australia.)