Any top who's worth fucking knows that spontaneity involves some risk, and if they're not prepared to cope with that, they need to grow up.
On the bottom's side of the equation, most bottoms have a sense of how they're bowels are at any given moment. Have you been holding in a dump for two hours? Was it a chocolate soft-serve factory the last time you went to the bathroom?
These are things any mildly conscious bottom should know going into any encounter such that if things start to go there, they can warn off the top, or if it might not be too bad can give them a simple, "I haven't cleaned out today, but we can give it a try..." Then if there's a little bit of mess, they were warned. And the grown-up top shouldn't have any issues having been warned.