Yes, it happened to me unfortunately.

It was in a mall. I was looking for toilets. Finally I found one, I opened the door and came in in a hurry, I entered a stall and, after a while I came out. And there, just ahead, two men standing in front of the wall, pissing, who turned their face to me. Yes, I was in the male toilet!

I could not say a word. Just tried to run back outside, but of course I took the wrong way: I enter in another space of the same male-toilet and I had to pass my way again, saying "sorry, sorry" like an idiot ...

mine wasn't a mistake. I did it intentionally because I am so curious how MALE restroom looks like. =) 

Years ago, I was visiting Ljubljana (Slovenia) for the first time, and had a little phrasebook of Slovene phrases. After lunch in a quiet cafe, I went to "spend a penny", as they say.  The toilet doors were marked:

M     Ž

Guess which words weren't in my phrasebook! :D

moški & ženske

Nope, but I used to be a lifeguard and a woman once walked into the male changing rooms while I was working. She gave me a look as if to say "what are you doing in here?" until she realised it was her that had made the mistake. Then her face turned as red as a beetroot.