Every edition has them. Rules that are obscure & forgotten to rules that may as well be written with invisible ink given how often players just ignore them. What are your big 5e serial offender rules?
- Dancing lights & other concentration spells that have the concentration stipulation ignored. IME the number one offender is people who could take the 1 hour non concentration light cantrip but instead take the 1 minute concentration dancing lights spell because it can be flown around & they can ignore the concentration flag on it.
- PHB195 covers Two Weapon Fighting, PHB165 the dual wielder feat, PHB72/91 the two weapon fighting fighting style. No you can not dual wield rapiers and add your dex bonus off hand without all three... also why the heck do so many people who are dual wield builds need to ask "how does the offhand attack work?"
- You only get half your hit dice back on a long rest(phb186), why does bob seem to spend all his hit dice every long rest despite dumping con?