When Stephanie takes the postcard from Michelle she holds it in her right hand and later puts her left on it. In the next shot, she is only holding it in her right hand again.
Even before 9/11, nobody has ever been allowed through a door marked "Authorized Personnel Only" and on board an airliner without a valid document of authorization, such as a passenger boarding pass or employee identification card.
Nicky and Alex (Rebecca and Jesse's twins) are born in #5.10 (Happy Birthday Babies, Part 2). In season 6, they are only supposed to be one year old, and yet in #6.15, Rebecca and Jesse are looking into preschools for them. Also, Nicky and Alex don't even go to preschool until the eighth season (#8.2).
There is only about a six week gap in the show between seasons 5 and 6, but Nicky and Alex seemingly age two full years in that time frame--they already have super long hair.