Continuity mistake: After the photographer asks Claire if they are still taking the picture Claire says no and puts mud on Gloria's belly. You see that its a round stain. In the shot after that the stain on Gloria's dress has become just a smear. This changes again in the shot after that. (00:20:05 - 00:20:40)
Continuity mistake: The scene in which the whole family were mud-fighting. Just after the camera man says "Excuse me, are we still doing this" look at Manny's shirt. It just has a small mud stain on the pocket, but as the scene continues the stain looks different with no reason and there is suddenly mud on Manny's face too.
Answer: Mitch is upset that his dad doesn't seem to support his getting married. When they talk about Jay's friends not coming to the wedding, Jay makes it seem like Mitch wouldn't want them to come anyways. Mitch sees it as it's actually that Jay doesn't want to invite his friends to his gay son's wedding because he's embarrassed and uncomfortable about having a gay son. Essentially Mitch sees it as if he was marrying a woman, Jay would be willing so spend money on the wedding, host it at the club, and invite all his friends to it.
Bishop73