I've had them do this for various things, nowhere near the 20% of the time they've called "the hallucination rate" but definitely once in a while if it doesn't know it will guess.

Which, to be fair, is what I've been doing for 40+ years in computer programming. When you have the ability to check if you've guessed right or wrong in a couple of seconds or even minutes, but researching it to "be sure" (and sometimes still get it wrong) before trying might take hours it's only natural / efficient to guess a few times before giving up and RTFM.