My Aunt Joan sent my mother an e-mail about a figure of speech called a paraprosdokian. The e-mail defined it as a figure of speech in which "the latter part of a sentence or phrase is surprising or unexpected in a way which causes the reader or listener to reframe or reinterpret the first part." I found the same text here. Examples of paraprosdokians:
I used to be indecisive. Now I'm not sure.
War doesn't determine who is right, but who is left.
A modest man, with very much to be modest about. - Winston Churchill on Clement Atlee
Searching for paraprosdokians online led me to this list of rhetorical terms from the University of Kentucky.