“If you choose an answer at random, what is your probability of being correct?” The question is, I think, pragmatically ambiguous. It features neither lexical nor structural ambiguity, yet the joke hinges on understanding the question in more than one way.
“Variable or non-standard realizations of inflectional morphology in English” sounds rather dry and academic, but the placement of suffixes within compound words or phrases can sound surprising and even amusing. Arnold Zwicky and Mark Liberman recently noted unusual verb conjugation. Non-standard pronouns can be equally interesting.