One of the mechanics I have seen incorrectly played around, including at high level play, is the Deadly keyword.
The Deadly keyword is relatively straightforward: a unit with the Deadly keyword will kill any unit to which it deals damage. The minimum amount of damage necessary to kill a unit, when a unit has the Deadly keyword, is a measly one damage, as evidenced by the cards Direfang Spider and Lumen Defender.
The first order of business regarding the Deadly keyword is to know where it fails. When a unit with the Deadly keyword enters combat with a unit that is immune to damage, such as an Ancient Terrazon, it will fail to kill the unit that is immune to damage. Similarly, when a Deadly unit has zero or less attack, it will also fail to register any damage, and thereby fail to apply its Deadly mechanic. Therefore, if you’re up against a deck with plenty of Deadly units, and you’re playing a deck full of justice units, one possible unit to counter this is the scion of Justice: Rolant, the Iron Fist. (As an aside, I would not recommend ever playing Rolant, as the possibility of him being a dead draw for too long in the game is too high to justify playing him.)
However, where the Deadly keyword gets tricky, is with two other synergistic keywords: overwhelm (which is exactly “trample” from Magic: the Gathering), and Quickdraw (which is the “first strike” ability from Magic: the Gathering, but ONLY when such a unit attacks, whether it attacks the player, or an enemy unit, if it gains the Killer ability).
In such instances, because of the Deadly keyword needing only 1 damage to kill a unit, the combination of Quickdraw and Deadly means that in order to even do damage in unit combat to a unit with the above two abilities, you would need to block it with more units than its attack stat. For instance, if you manage to damage your opponent with Gorgon Swiftblade and trigger its Infiltrate ability, your opponent would only be able to chump-block it unless it is blocked by three or more units. Along the same mechanical line, the card Ashara, the Deadshot can kill up to six blockers without taking any retaliatory damage.
Moving onto a more common combination in constructed, the combination of Overwhelm and Deadly, means that blocking Champion of Chaos with any card that isn’t Cirso, the Great Glutton (because of Cirso’s ability) will mean that the blocker dies, and that Champion of Chaos deals 4 damage to the opponent (assuming it’s fully buffed but not equipped). That is, for each unit that blocks a fully buffed champion of chaos, that unit dies, and you save one damage that the Champion must deal to your blocking unit before dealing the Overwhelm damage to you. Another mechanically identical interaction would result in equipping a Statuary Maiden or Steward of the Past with a Lethrai Falchion, or equipping any unit with the Overwhelm ability with the Venomfang Dagger, which is usually an exceptionally bad card, regardless of the format.
However, one neat draft-tier combo: if you equip a Stormcaller with a Venomfang Dagger, its active ability means that rather than dealing one damage to an enemy unit, it can outright kill an enemy unit instead.
Lastly, with more pertinence to draft, a Champion of Chaos attacking into an Ancient Terrazon (that is, the 7/7 immune to damage unit) will not be able to push through damage. That is, indestructability blocks all of a Deadly-overwhelm combo’s damage, not just the necessary one point of damage.
I hope you learned something about the Deadly keyword. Thanks for reading.