The enemy attack bug in the NES and other early versions of Final Fantasy changes the element of regular enemy attacks that can inflict status effects. Normally, the attack is intended to be of the same element as the status, eg. a poison-inflicting attack would be of the poison element. The bug instead results in the attack being of the same element or elements as the attacker's weaknesses; if the attacker has no weakness, the attack becomes non-elemental.
For example, the Aegis Shield should provide the wielder with resistance to the Poison condition inflicted by a Green Slimes regular attacks, as they should be of the Poison element. Instead they are of the Fire and Ice element, the same as the Slime's weakness, and bypass the Aegis Shield's protection, but a Flame Shield or Ice Shield can instead resist it by providing resistance to their respective elements.
Alternatively, Mindflayers are made especially dangerous, since their attacks have an instant death side-effect that bypasses protections such the the Protect Ring. And as Mindflayers have no weaknesses, the attack is rendered non-elemental and cannot be blocked by anything.
This bug was corrected in all subsequent ports and remakes.