Very, very small details. That and the fact that the paper it was printed on was a bit too slick. As for the details, there are a few small messups. The one I can see every time is this: Look at the right half of a new series $100 bill to the first N in United States of America. The upper gap in the N should be all black. In the supernote, there is a little sliver of white in there.
Besides this, sometimes the notes seem just a little bright in light. Also, the serial numbers aren’t always even – one number is higher or lower than the the one next to it. Also, the thickness of the lines making up the serial number can be uneven too.