I see people breaking rules all the time. For this particular offender on my way home yesterday, his "ride super fast and try to drop the guy in matching kit whilst looking backward erratically" speed was strikingly similar to my "riding with no hands while trying to fish a camera/phone out of my backpack" speed, so he got documented.
Don't do this:
Of course this guy is breaking all sorts of rules (backpack, non-functioning blinky light, dork disk (not pictured), hairy legs, MTB jersey, 1980's bike shorts with neon yellow and purple stripes. All of those would be forgivable since he obviously isn't a roadie, he's a commuter, albeit a commuter desperately trying to drop a roadie. . .
But under NO circumstances should ANYBODY be allowed to rock the back gap. Just don't people. The LAST thing I want to see on my leisurely ride home is your hairy, sweaty assback. It's only made worse by the part where you're desperately trying to stay in front of me, and my mandated recovery wattage isn't enough to drop you.
Related note: Don't try to drop people on the bike path or on city streets. It's stupid, unsafe, and worst of all, it makes you look desperate. If you want to race, you know where to sign up. If you want to be competitive with other cyclists on the road, even that has a place, but that place is on the training hill, not on your commute home. Definitely not on mine.