In the ending scene when Robert Deniro is watching the video of Ben flushing the toilet, Ben talks about how Denny smokes pot. But, if you remember, Ben doesn't meet Denny until the next morning. You know that the first time he meets Denny is in the morning, because he says it the next morning.
Actually, the scene where he talks about Denny took place ANYTIME AFTER he met Denny. He doesn't talk about him in the bathroom scene. He talks about it into a camera in the den or something. Bob-Jack had lots of different footage of Focker, and was just viewing the "Denny Smokes Pot" take after he watched the bathroom scene. Look at the background. I CAN see how you'd get confused though. If I hadn't seen the movie only hours ago, I would've believed you.
Actually, I did notice this and I was going to add it. If you notice he is wearing the same pajamas he wore the first night. Also, why would he wear those pajamas again after he got his bag back? It also happened right after the bathroom scene on the tape. So he didn't really know Denny until the next day.
Actually, Greg didn't get his bag back until the night he left... He got A bag, but it wasn't his, and so that would explain the pajamas thing... He never had his bag in that house until the very end when he came back for Pam...