Rules we all think are true, but not written anywhere?

Talking to superior ranked NCOs I'd often a unit policy not just regs. Go to the 82nd, 101st or the Ranger Regt and you'll see it everywhere in garrison.

US Army Europe 1981 Everyone who earns Honor Graduate at the NCO Academy gets put to the end of the duty roster and is awarded the Commendation Medal... that happened in my unit until I went to the NCO Academy and earned Honor Graduate.

How about Officers don't ware ribbons on their shirt, that's an Enlisted thing.
Or
Officers don't remove their abu/acu top while working, only Enlisted.

In the Army, many still believe no cover requires if "under cover" (e.g. when pumping gas). The regulation now specifies not worn while "indoors" (AR 670-1, para 3-7) for which at the gas pump would not apply.

19D scouts can't go to sniper school...

Officers don't wear ribbons in blues.

I am not bothering to check the AFIs on this, but I have never seen an officer wear their ribbons on a random or scheduled blues day. Granted only half of my assignments have been in a place that has blues days, but still it was a norm that I noticed across assignments to multiple commands.

When I was a young troop in the 1970s there was some folks in the Army that believed you could be Court Martialed for carrying and umbrella. I carried mine because it beat getting wet. I guess I was a non-conformist.