Strange Unique items were created due to a bug introduced into TF2 with the Scream Fortress 2016 update. The nature of the bug was that any item that should have been created with Strange quality ended up being created as a Strange Unique.

The majority of items that were produced from this glitch originated from crate unboxings, but the glitch affected all items that should have been Strange, including MvM rewards. A handful of Strange Unique Australiums and Botkillers were created as a result.

There are 423 known instances of 'Strange Unique' items.

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