If you're trying to pass as a man, people will call you ma'am. In my case, sometimes it's "sir" [long pause as eyes roam frantically around my body, looking for a clue; relieved, catch on the booblets], then, with self-satisfaction, "excuse me, I'm sorry, ma'am!" [because clearly any biological woman would be offended to have someone mistake hir for a man, even if s/he'd been shopping in the men's department for 15 years]). It's funny because I don't recall anyone ever looking at my booblets before I started trying to pass.
Special note to fags: stop calling butches ladies. It's annoying, and we're a lot stronger than you.
When I break down and go into the women's room (usually at the airport), some woman-passing-as-a-transvestite will shriek and pronounce that I'm in the wrong restroom. (I'm embarrassed to admit it, but someone once told me to tell my mother I was too old to be in the ladies' room. But that was years ago.)
Restrooms are the only trans topic anyone could call overdone, so I won't dwell there for long. But I do have a few quick complaints about the men's room. No one likes to be presumed guilty until proven innocent—of crapping. Oh, and guys, did you know women's rooms sometimes have fancy fountains and stuff? That's because they're not always filthy.
But back to Murphy's Law. When you identify as butch, somebody will say "you're not that butch." Let's call this butch denial, which is a lot like climate change denial—it's not that hot! Denial in both cases is an attempt to remain on the firm ground of the status quo, without having to change the way you think or act. (Pretty much just to be mean, I'm linking to Senator James Inhofe's [PDF] magnum opus of climate change denial. And to Mother Jones' proof that ExxonMobile funds the scientists Inhofe cites.)
To get butch deniers to admit that it's possible to be genuinely masculine in a female body, you'd have to be a catastrophe of butchness—6'5" with a body builder's physique and mean to boot: Hothead Paisan on meth and testosterone. But at that point, it's no longer a question of masculinity in a female body; the body has been brought in to line. And, what's more, this Incredible Hulk/Hothead Paisan hybrid would be such a laughable stereotype that the deniers wouldn't have to take hir seriously. So no need to change one's thinking--or lack thereof--about gender after all. Phew.
As for femmes, Murphy's Law says that no matter how many times they come out to straight people, said people will "forget" that they are, in fact, lesbians. Now, lady-lovin' butches and trannies might not forget repeatedly, but we've been known to assume the worst (heterosexuality) until told otherwise. Sorry ladies.
And now, for two "user-driven content" opportunities (it's the interweb, after all!): femmes, post comments with your experiences with Murphy's Law of Gender and Sexuality. And, butches/transmen, in the spirit of Holla Back, send me pictures of the women-passing-as-transvestites, Midwestern butch straight ladies, and others who have challenged your right to be in the women's room. Or wiener dudes who've challenged your right to be in the men's room. (Let's face it, at this point, you all have my email address!)