My personal story, this isn't much for a rule but more of a wargear...I forgot my Obliterators has Powerfists for a a few months and couldn't figure out why they weren't that great in
CC. A friend of mine used to think extra armor added +1 to the armour value. I was playing my 2nd game ever as
CSM with a Sorcerer with Bolt of change, and a Chaos Lord with a Combi Melta (I didn't know what I was doing at the time and I think that was all the Anti-Armour I had). He gave his Furioso Dreadnought wings and Extra Armor claiming it boosted his armor value to 14 meaning it was literally a

ing flying Land Raider almost. He preceded to kill everything I had that could so much as scratch it and claim the easiest win ever.
Edit: I got one more. When my group of friends was still learning...we thought
FNP worked through everything. Including
AP 1/2, Power Weapons, Fists...you name it. It was quite the funny story when we learned how wrong we were afterwards.