Can’t say flipflops bother me but I’d go with the earlier poster who brought up crocs. Those should be punished by a bullet in the head and a shallow grave if the wearer is over, 13-14. You could JUST ABOUT get away with them as an adult if you’re on a rocky beach but for that I’d probably just hand out some sort of maiming rather than execution.
Oh and if you’re wearing crocs on the plane and then tell your neighbour in a loud voice how comfortable and convenient they are for air travel whilst the rancid odour is spreading around the aircraft, it’d be the old hung, drawn and quartered for you.