The Null Device
The EURion constellation
A PDF document showing the "EURion constellation"
; this is a constellation of five 1mm circles found on European and British banknotes, and recognised by colour photocopiers. (The piece doesn't show exactly which five circles are the constellation, though if it's there, it shouldn't be too hard to find it.) I'm not sure whether the pattern used on US banknotes (and now recognised by commercial image-processing software) is the same or different, or what other anti-copying patterns are used on other currency. Though if each country had its own, it would use a lot of CPU cycles to detect. (via jwz's comments)
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