Don't assume, check.
I always remember the Airclues cartoon of that statement that hung in crew rooms of the RAF years ago. There was a caricature of a Winco giving a lecture to a group of pilot officers on flight safety and his flies were undone. Silly but effective, and it has saved me a couple of times over the last many years.
Meaning at my age I still come out of the lav with my flies undone, but at least I do a careful preflight.