The Error Types That Defy Classification

An Unidentified Fascinating Oddity (UFO) is a die variety that doesn't fit into any established error category. Raised bumps, unexplained marks, unusual impressions — features that reproduce consistently on every coin from that die, confirming they're genuine die varieties, but whose origin is unknown or debated.

The UFO designation is an honest acknowledgment: "We know this is real, but we don't know exactly how it happened."

How UFOs Form (Maybe)

Because they're by definition unclassified, potential causes span a wide range:

Contamination during hubbing — Foreign material trapped between hub and die gets pressed in under hundreds of tons of pressure
Incomplete die polishing — Aggressive polishing creates artifacts that don't fit neat categories
Die contact events — A die strikes a collar fragment, broken feed finger, or machinery part
Multi-cause features — A die that was clashed, polished, re-hubbed, and cracked produces compound anomalies

The Wisconsin Quarter "Extra Leaf"

The 2004-D Wisconsin quarter "extra leaf" varieties — showing additional leaf-like features on the reverse corn ear — were initially classified as UFOs before analysis determined they resulted from die gouges. This is the UFO lifecycle: a mystery is cataloged, studied, and eventually reclassified once the mechanism is understood. Today's UFO can become tomorrow's recognized category.

Discussion

Have you ever found a coin with a feature you couldn't classify?
What's the most puzzling die variety you've encountered?
Do you think most UFOs will eventually be explained, or are some truly mysterious?
Post your unidentified finds here — let's investigate together!

UFO Errors

UFOs: Coin Mysteries Waiting to Be Solved