Yes, something similar happened to me. A regular I'd been seeing for years, always used a condom, although she did owo.
Then during the last visit, she asked if I was ready to have sex, and just sat straight on my cock, without protection. I removed myself immediately, and we chatted about it at the end of the session. She said she must have got carried away, because she always used protection.
That sounds unlikely to me, so I haven't seen her since - it was about a year ago now.
Got tested after the requisite wait, and was clear, thankfully.