How printers and Photoshop detect and refuse to process images of currency
When I was in college (99-2000ish, maybe?), one of my roommates was an art major with a graphic design bent, and he had this high end printer and would copy and print $1 bills and take 20 at a time over to the vending machines around the corner from our apartments once a week or so.  After about a month of doing this, the machines disappeared.Of course, he's the same roommate who later got arrested for wire fraud and grand theft.