We got "caught" using last year's mugs, but it wasn't until the last couple of days of the cruise. According to the bartender there is a date marking on the cup that they are supposed to look for. FYI the cost ofthemug for the 7 day has gone up to $85...ick!
We purchased our trip from AAA back in 2002. At that time if you were a AAA member you received free mugs for each person in your party. Therefore, we got 5 free mugs! Saved us $50 which we put towards the pictures they take!