"Worst" case of not knowing the rules in terms of impacting play speed was trying to have one player run another player's paragon 4e character when they were out. The misunderstandings, retcons because "oh, that's how it works", and time spent were ridiculous. This happened on a regular basis in one game, where the character was still there (and needed) regardless if the player could make it. Not really too much of a player's fault, just so many individual powers to evaluate that were close but not the same.
Worst case in the players should know better is I've gamed with people who need to do the math every single time. A melee combatant that needed to redo the to-hit and damage math every roll, even on the second roll of an attack sequence. Or those that can't get the listed on the character sheet skill bonuses correct. They try to refigure them each time, use the Save instead of the Ability mod (which may already include proficiency), etc.
Players regularly forget rules that don't come up very often - like only restoring half your HD on a long rest. Or disadvantage on Stealth from some types of armor.
Though the most common one is no one playing a caster ever, ever seems to remember to roll for Concentration, including the DM. I am in two campaigns with heavy overlap in players, and I swear that I am the only one in that group to ever, ever roll it when taking damage. And even an AL-type game I was in it was mostly forgotten.