Months back, I arranged a game with my old AD&D GM to join my current 5e D&D group. I'd pitched them an "old school" 2e experience and they seemed interested. Everyone had their characters made and we started the first session. Pretty quickly, the nightmares began.
"Wait, so AC goes down, right?"
"Yes, that's correct."
"Then that +1 chain mail we found is cursed?"
"No no, +1 bonuses subtract from AC."
(Another player) "Wait, that can't be right, my Dexterity gives me a -2 to AC, that's good, isn't it?"
"Yes, it is..."
(Third player) "I'm still trying to understand why everything isn't a penalty if a lower Thac0 is good. Like, my Thac0 is 19, but I get +2 to hit from my Strength and +1 to hit from my Weapon Specialization. Wouldn't it be easier to say my Thac0 is 16?"
(AD&D friend) "No, see, you add the bonus to the die roll, then you compare that to the Thac0."
(Third player) "But...look, AC goes down, right? So why not have low numbers be good on the d20 and subtract all modifiers? So instead of +3 to hit, I'd have -3 to hit, and if I want to hit AC 3, then I need to roll a 6 on the die or less."
(AD&D friend) "Uh..."
(Fourth player, chiming in) "Then you'd have to make ability checks work the same way."
"Well actually, that is how they work."
(Fourth player) "...so wait. We roll high to attack to hit a low number, but low on skill checks? This is all backwards!"
(AD&D friend, not getting everyone's frustration) "Well, sometimes you roll d% too!"
It was decided after that session to go back to 5e, and my AD&D friend has been put out about it ever since.