Raise your hand if you've shit yourself in the past month. I have not (although if I had said two months it would be different). I've had lots of SCARES (mostly farts that turned evil at the end), but no real accidents. Enter UrbanUndercover (currently a nearly-funded Kickstarter campaign), a line of women's underwear with a hidden pocket in the rear for carrying a spare pair in case you get your period or something else womanly happens that I don't understand. You don't have to carry underwear in it though, you can also carry tampons, wet wipes, cash and ID -- anything that will fit. But please, no pets. Personally, I don't carry a spare pair of boxers around because if I make a mess in the back of my underwear I am NOT putting a new pair on. That is a clear sign to call it a day, go home, and eat a pint (read: quart) of ice cream in the shower. I've told you about the time I shit my pants on the playground right? "Too many times." I had to run to the bathroom cupping the back of my shorts so the weight of the turd wouldn't pull them down. On a quiet night I can still hear all the other kids laughing.
Thanks to lilco, who agrees if you don't wear underwear in the first place, you'll never need a second pair.