Well, although the Charlton guide use to say that no double denomination is been found, I do not believe that it says it anymore. Seems to me one came to light about 2 years ago, but Tom might have more details.
Technically, this is possible, because printers use a number of sheets various parts of the printing process for "setup sheets". It is conceiveable that a setup sheet from a $5 press run ended up being used as a sheet up sheet for a $10 (you know, conserve the paper) and then managed to slip through detection. Now, I'm not saying it a likely possibility as I would have thought it would be easy to detect, but again, it is possible. It could also be a "back door job" where somebody in the plant "assisted" it's introduction to the market.
As well, it wouldn't be hard to just run a note through an inkjet printer either...
So, what I'm saying is that it is "possible", just very unlikely. I'd kill to put my grubby little hands on it (and my loop), but Tom would be better.