Barbed devils have a passive physical attack that involves you getting pierced by their barbs. Possibly if you are wearing body armor and/or gloves, or wielding a two-handed weapon with longer reach, the passive attack may not always hit you. (Polearms should never incur this attack, but the way passive attacks work by default should already ensure this.)

Wielded wood and plastic weapons (and possibly glass) block passive shock attacks, since they are non-conductive. Plastic could block 100% of the time and other materials less so, mainly to give a fringe benefit to a material that’s normally pretty underpowered.

The passive attack only gets blocked if you actually used the weapon to make your attack: if you’re polymorphed into something with a weapon attack and a bite attack, for instance, the bite will still subject you to the passive shock.

Applying a stethoscope to a monster that has a rusting or corroding passive attack erodes it appropriately.

Juiblex has a passive acid attack, and a passive acid explosion attack (AT_BOOM AD_ACID, which triggers on death).

Hitting monsters that have a passive shock attack with something made of copper, silver, or gold increases the amount of shock damage.

A sessile invisible monster with a nasty passive attack.

Hitting monsters with a passive shock attack doesn’t hurt you if you’re hitting with a non-metallic weapon or your hands with non-metallic gloves on.

Warn the player before they melee a monster with passive attacks.