This is only a little thing, but it surprised me. Another SSiWer (@EsmBoone) and I were manning the Welsh Society of Manitoba table in the UK pavilion at a cultural festival in Winnipeg, Canada last week. (Sidenote: If you're interested, the festival is called Folklorama, and it's the largest and longest running cultural festival of its kind in the world.)
Anyway, I was sitting there one day towards the end of the week, and some member of the public came up to me and said (in a VERY mocking tone), "Yeah, but I'll bet you can't speak Welsh." To which my mouth replied (without at all consulting my brain), "Yndw, dw i'n siarad Cymraeg!" It actually startled me - my mouth deciding that I wasn't going to say, "I'm a learner" or "I speak a bit"; but at that point I was committed, so I just smiled innocently at him and carried on, "Dach chi'n siarad Cymraeg?"
It turns out that the man who asked me couldn't speak Welsh at all, so I think I startled him even more than myself. ...Karma?