Here's a dollar bill I got over the counter one day, back when I worked for the Post Office:
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Note that the serial numbers are smeared (the red one on the left doesn't show up as good as the blue one).
Yeah, I know what someone is going to say -- "You can do a lot of different things with chemicals..."
Now, look at the back:
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As you will see, the serial number of the note printed underneath also had a very wet set of serial numbers.
I would have loved to have this bundle of notes...