Some people (very cleverly) did the math, and ordered ridiculous numbers of dollar coins on their airline-affiliated credit cards. When the dollar coins would arrive, the buyers would take them back to their bank, depositing the money in their account.
In other words, these people were getting thousands of dollars' worth of frequent flyer miles each week, for absolutely free. The Mint found out about this when banks started sending them enormous numbers of dollar coins, unopened, for storage, and that a very small number of customers were ordering a very large number of dollar coins.
The full (and very entertaining) story was done by NPR's Planet Money podcast:
http://www.npr.org/blogs/money/2011/07/13/137795995/how-freq...