A verse can conform
to one well-known form or more,
but one is the norm.
(I want the "or" and "more" to kind of cascade at the end there, each being on a subsequent line but at the appropriate alignment for the line above, as a compromise between the limerick's demand that they should form the third and fourth lines, and the haiku's demand that they should be part of the middle one. But it doesn't look like blogger will do that.)
Anyway. It turns out I wasn't even the first person to think of this. Some of those are even reasonably funny -- I like the E.O. Parrott one.