Despite getting applied over 12 hours apart, the ink on Rachel and Ross' faces fades at the same time.
When Monica and the others are having breakfast at the beginning of the episode, Monica's hair is way longer than at the end of season 5 but it's supposed to be the morning after.
Joey asks Phoebe what time their flight leaves the morning after, yet Phoebe told the lurker she was leaving on Monday. Ross and Rachel arrived in Las Vegas Saturday night and stayed in the room before getting married. This would make it Sunday morning, and when Joey and Phoebe wake up in the taxi it would be Monday morning.
When Joey and Chandler hug at breakfast, the ghost reflection of a white shirt like the waitress can be seen in the glass behind them, but in the extended camera angle, both staff members are across the room and no one is near them.
When Joey and Phoebe rush into the chapel, their mouths don't match what they're saying.
At the end of series five Rachel and Ross get drunk in Las Vegas because Ross has used a permanent marker pen on Rachel's face, which they are told by the manufacturers cannot be removed and will only fade with time. She uses the same pen on him. At the beginning of series six they wake up together with all the markings on their faces but when they arrive downstairs for breakfast all the markings have been washed off completely. This is a major plot hole because if the markings were washable they would never have gotten drunk together and ended up marrying.