The backpack that {falls in the water | gets eaten by a bear | spontaneously bursts into flames} will be the one containing all the TP.

Much suffering can be avoided by the simple precaution of bringing a roll of Emergency Toilet Paper stored in a zip-lock bag in a side pocket. To score experience points with the rest of your party, don't tell anyone about your secret stash. Then, when the inevitable disaster occurs, let them wail and gnash their teeth for a bit, then reveal your Emergency TP and bask in their adoration.

Emergency TP is also a good idea if you live communally with roommates. Inevitably, YOU will be the only person who ever buys TP BEFORE the last roll runs out. It saves a lot of grief (and paper towels/Kleenex/old term papers) if you stash one or two rolls away, and produce them only grudgingly, after reminding your buddies that it's their turn to be seen purchasing butt-wipe at the grocery store. If your "buddies" are really "dirty assholes" (so to speak), these rolls can be kept hidden and used only by you, while they rasp themselves on shiny Pizza Hut fliers.