When an imposter is spotted because the person he's impersonating is in the group/room with, or is himself, the person they're trying to deceive. In its basic form, the conversation will go like this:
"Jonesy" comes from this page (and the one before it) of The Adventures of Dr. McNinja #1.
If you have a good enough disguise, you might still be able to get away with bumping into the person you're [...] [tvtropes.org]