I went out en femme for Halloween, which is of course not really a costume for me, but it was the only night I could be sure that nobody would give me guff.
Anyway, I had a sort of startling realization that I'm fairly sure I was passing.  First, without even having any makeup on I got carded at the grocery store when picking up a bit to drink.  When she saw my picture with short-cropped hair and a beard, she gave out a good giggle.  There was just something about that giggle... I didn't have any makeup on yet so I had a visible facial hair shadow, but I think she read me as female initially, then it clicked when she saw my ID.
Later on, with makeup on, I ducked into a bar to use the restroom and to avoid complications I just went to the men's room.  Hardly odd on Halloween.  So, when I came out of the restroom I heard a guy go, "Whoa! That chick just came out of the men's room!"  That felt pretty good.