M+S, ladies'underwear section. It's all cubicles and to be fair, nowhere does it say that it is a women-only changing room, but I think it's likely to be seen as such by most users since it's in the bra-and-knicker-selling section of the shop and there are other changing rooms elsewhere!
A woman came out of one of the changing rooms whilst I was queueing and beckoned a man in. I said, I'm sorry, I don't think you should go in there, this is for women trying on underwear. The woman queried this but I stuck to my guns and the woman behind me in the queue backed me up; the man apologised and left. (It probably helped that they sounded as though English wasn't their first language so they may not have known the "rules"! I think it was a genuine misreading/mistake on their part.)
Anyway, no harm done, but it made me think - that was easy, but if it had been a very obvious man-dressed-as-a-woman, I'm far from sure that I would have felt able to speak out.
In that situation (the man-dressed-as-woman), would anyone here have said anything?