1. I asked it to show me the derivation of a formula for the efficiency of Stop-and-Wait ARQ and it seemed to do it, but a day later, I realised that in one of the steps, it just made a term vanish to get to the next step. Obviously, I should have verified more carefully, but when I asked it to spot the mistake in that step, it did the same thing twice more with bs explanations of how the term is absorbed.
2. I asked it to provide me syllogisms that I could practice proving. An overwhelming number of the syllogisms it gave me were inconsistent and did not hold. This surprised me more because syllogisms are about the most structured arguments you can find, having been formalized centuries ago and discussed extensively since then. In this case, asking it to walk step-by-step actually fixed the issue.
Both of these were done on the free plan of ChatGPT, but I can remember if it was 4o or 4.