The truth is, I've been waiting a while for the opportunity to make this joke.

The Wug Test was apparently designed so that "Each "target" word was a made-up (but plausible-sounding) pseudoword, so that the child cannot have heard it before." But this can't possibly be true, because the word "wug" pops up in every linguistics book.

— Joel Swagman (@JoelswagmanJoel) February 27, 2021

Wikipedia source here.