@JeremyECrawford wait, so Dex modifiers don’t count when wearing heavy armour? Even negative mods? Heavy armour removes clumsiness?

— Jesse Bruce Clay (@Dad_On_Arrival) November 7, 2017

The rule on heavy armor does make your Dexterity modifier irrelevant to your AC (see PH, 145). #DnD https://t.co/e2rgd6V7Ud

— Jeremy Crawford (@JeremyECrawford) November 7, 2017

I’ve noticed a few instances in published adventures where creatures have had a Dex penalty applied to their AC despite heavy armor. Monsters can have tweaks that break the rules, however #DnD #dnd5e

— Marco Fossati (@haelyn78) November 7, 2017

Yes, monster stat blocks often contain ad hoc modifiers.

— Jeremy Crawford (@JeremyECrawford) November 7, 2017

That’s not what I meant. Something like “This monster wears plate but its AC is only 17 because …” The Dungeon Master's Guide makes it clear that monster stats are often ad hoc. See "Creating a Monster" (DMG 273–83).

— Jeremy Crawford (@JeremyECrawford) November 8, 2017