A roughly biannual phenomenon, Random Anus Pain will occur (as the name suggests) for no apparent reason.
Usually happens when the sufferer is in the car, but not always. There is also an ensuing terror that 'This is it!' followed by perhaps several minutes of hyperventilating and the urge to immediately call someone and tell them about it.
It is thought that Random Anus Pain is similar to the body's 'falling off a mountain' flinch reaction when you are about to fall asleep. Except R.A.P. is your body's way of telling you that it can ruin you at any given moment, so you better start appreciating it.