I am in the habit of exiting the lavatory before I have completely buttoned up my trousers and buckled my belt when at home. 99% of the time I remember to limit this to my own personal space, but there have been a few moments of pub and work-based forgetfulness. Many other men I know or have seen in public seem to have a similar approach.
Do you? Interesting question, I know. Feel free to pad your answer out with an anecdote about a time you suffered a trouser or belt-related embarassment.