A shift (dream, mental, phantom or otherwise) that is of an animal which is not the shifters wereside. (For example: a tiger therian experiencing a wolf m-shift.)
These shifts are usually one-offs and have little if any meaning.
It's possible that they do indeed have meaning... but the meaning is not necessarily that you have "acquired a new wereside". It could be something else that causes this, but it may be something with meaning worth exploring.
Going along Coyote's mention above, I'd even go so far as to suggest that a Cameo shift may be related to one's personal totems — it isn't unheard of for a totem to act strongly when it can't get a point across in any other way.