something unusual -- perhaps worthy of collecting

a broad category of unusual variations. Examples: stamps used before their first day of issue, errors in postmarks, and semi-official separations such as the Kansas City roulettes.

when something unusual happens to a coin it is sometimes called an oddity.

a strange attitude or habit

The quality or state of being odd; singularity; queerness; peculiarity; as, oddity of dress, manners, and the like.

Any single matchcover that belongs to a set by virtue of its origin, but doesn't look anything like the other members of the set.

eccentricity that is not easily explained

a better description for me

That which is odd; as, a collection of oddities.