If someone were to ask me, "Aren't you in the wrong loo?", I'd reply, "Me? I thought you were. The gents' is next door, isn't it?"
My favorite loo story: After a pride parade last year, all of us went to this big hotel for a few drinks. At one point I went into the loo. Somehow I was the only one in there. Huge hotel, people all over the place, yet I was the only one in the loo.
While I was taking care of my business, I heard somebody come in. From the sounds they were making, I told myself, "Must be the cleaning lady."
When I came out of the stall, I saw that I was wrong. It was the cleaning guy. He immediately went all apologetic: "Sorry! I didn't know you were in there."
I felt like screaming at him, "Get the ***** out of here, you ***** pervert!" But I just smiled and assured him he was OK. I hugely enjoyed the irony of the situation.