okay here's one i'm actually serious about that's kind of controversial: Koko the gorilla did not know sign language. in fact she did not know any language. she was very intelligent and emotionally deep, but she did not do what humans do when we communicate with each other through spoken or signed words. at most she repeated some actions that got her some reward, but she did not associate the actions with any meaning beyond the reward. her trainer did not know ASL and did not bother properly codifying the reduced version she supposedly taught Koko. instead she convinced herself of her conclusion, and having done so, worked to imbue any pseudo-signing behaviors by Koko with linguistic meanings whether they made sense or not.
I believe pretty much the same thing about everyone on youtube and tiktok who made those button boards that their dog or cat presses to "talk." i think these cases and claims are worth investigating scientifically, but so far no one has definitively demonstrated what would be a very surprising conclusion i especially find it suspect when they say things like "so far Grover has learned over 80 words!" and what they mean is their golden retriever has pressed over 80 buttons that said a word that the owner could come up with some explanation for.