It might be a bit stupid, but I went on a student exchange to France 14 years ago. And my guest family's daughter had a friend called Clemont (a girl, for context). And I was distracted and only half listening when she introduced herself so I just blanked, pointed at her and went "Klaus?". That was apparently highly offensive, as she never spoke to me again. And my two childhood friends, who bore witness, forever teased me about it by calling every single partner and ex partner I have and had "Klaus."