Personal experience using agents and seeing this happen frequently.

Try it yourself. Get one to hallucinate, then paste that text into a new window and ask it to verify the facts.

EDIT:

Also - Cohen, Hamri, Geva & Globerson, "LM vs LM: Detecting Factual Errors via Cross Examination": Cross-examination "detects over 70% of the incorrect claims while maintaining a high precision of >80%".

So 70% for ANY error, not just hallucinations.