If I had to guess, what you are most likely seeing is the rule:
While encountering a card, each player may play no more than 1 card of each type during each step.
When in a combat check, that "step" includes all of:
Determine which die you’re using.
Determine the difculty.
Play cards and use powers that affect the check (optional).
Assemble your dice.
Attempt the roll.
Take damage if you fail a check to defeat a monster.
For example, if you use an item during your combat check, you can't use a different item's powers to reduce damage if you fail the check.
except thats wrong. if say ezren uses lightning bolt to attack, he can still use arcane armor when he takes damage because attack step and damage step are two different instances. also, the example is that i can't say, stabby with knife or roast with fire and then use sihedron medallion or ammy of life, or x other item that says reveal/discard this to reduce damage. completely different card types to boot. and sometimes its just damage from a trap that doesnt have a tag line saying it cant be reduced. so i'm just stuck taking damage when i shouldnt