I haven't experienced chatgpt explicitly tell me "I don't know", but it has provided me with factually incorrect answers pertaining to the functionality of an appliance and associated software. Further probing and I was advised to contact IBM support.... lol.

My usage has been confined to specific technical inquiries and chatgpts benefit has been colossal in saving me the time of sifting through decade(s) of documentation, forum posts, bug reports, kb articles, etc.