Hallucination
Output that is fluent and plausible but factually wrong or invented.
Hallucination is not a bug that will be patched out; it is a consequence of models optimising for likely continuations. It becomes dangerous when it is automated and nobody reads the output.
Mitigations that work in practice: ground answers in retrieved sources, ask for citations, require the model to say when it does not know, and design a UI that makes verification cheap.