When I was on my mission, my companion and I went over to a бабушка’s house one time to do some service of some kind (it’s been many years and I don’t remember what exactly we were helping her with). After we finished up, she said something like “thanks! I really appreciate it, maybe I’ll ask you to come help again sometime” or something to that effect. Being a millennial, I would always answer “thanks” with не за что, which means approximately “it’s nothing,” like “it’s no trouble at all” or “it’s my pleasure.” My companion was newer in country and didn’t speak Russian particularly well, and instead said ни за что, which in an American accent sounds almost exactly the same, except the stress is moved, which instead means something like “not for anything” or “under no circumstances.” Luckily, I caught what had happened pretty quickly and smoothed things over, because she positively blanched when he told her we’d never come back to help her again no matter how much she paid us.

edit: I’m pretty sure he was the same companion who said he was sterile (я холодной) when he meant he was cold (мне холодно).