Carrie has been sick at one point in the flashback episode (Meet By-Product) where Carrie and Doug meet for the first time. She and Doug are sitting on the couch and are starting to hit it off. As they are talking she is holding a glass, which somehow mysteriously disappears.

I saw this episode being filmed and this mistake is probably due to editing, as I seem to remember her having the line "Thanks for the Strawberry Quik" that didn't make it into the episode that aired.