If you take half damage from an odd number, take the smallest number of the two. (e.g. 19 damage halved, 9+10 = 19, select 9.)
Stealth checks: /roll 20, plate wearers have disadvantage and negative 5, cloth/DH nothing special, rogues get +5
Pounce/grapple is a hand-to-hand check vs the target. Every turn (including defense), grappled target must roll another hand-to-hand check against opponent to see if grapple is broken. While grappling, both parties are unable to act further or roll defense, although base defense modifiers are taken into account.
Critical Failure (rolling a 1) has penalties as follows:
Attack roll: Your opponent parries your attack and counters, dealing 1d12 damage. (you /roll 12 and take that number as damage)
Action attempt: Depending on what non-attack action is attempted (polymorph, freezing in ice, using something, etc), up to DM discretion the blowback.
Defense roll: You are hit critically, taking double the damage.
Stealth roll: You tumble over something, alerting nearby enemies to your presence. Note: Alerted enemies may not necessarily notice those who didn't fail the stealth check, DMs must take into account player positioning.
Feral druid claw attacks are considered hand-to-hand.
Illidari may consume the soul of 1 fallen demon mob for 10 HP. Multiple Illidari cannot heal off of the same soul.
If players are struck by an enemy Nat 20, they do not get to roll defense. Instead, only that particular defense skill's points is counted in damage negation.
Consumable items that do damage do not have their damage increased by Heroic Abilities. For example, a Scroll that does 20 DMG will still only do 20 DMG even during a Hero Ability that makes you do double damage, because it was only enchanted to do 20 DMG. However, if a Hero Ability causes a target to take double damage from all sources, the Scroll would do double damage.