"This card is unaffected by the effects of Spell/Trap cards..."

So, I was dueling the other day on DN and got into an argument with a guy (which I ultimately won, still lost the duel though through a Deck Out). The argument was about whether he would take battle damage.

Here's the scenario. My opponent activated a Trap card, can't remember the name of it, that made his monster unaffected by other Spell or Trap cards for the rest of that turn. He attacked my Mask of Darkness, which was in attack mode due to me Flip Summoning it, and I activated my Dimension Wall Trap card, to transfer the battle damage I would've taken to his Life Points. The argument ensued, with him claiming that Dimension Wall didn't affect his monster. Technically right, but Dimension Wall wouldn't have affected it anyway.

The thing is, cards that say "This card is unaffected by the effects of Spell and Trap cards..." or "The monster targeted with this card in unaffected by the effects of other Spell and Trap cards..." only work on Spells or Traps that would normally affect the monster. The effect would allow the monster to bypass a Swords of Revealing Light or a Mirror Force. It would stop the effect of a Book of Moon when activated in chain. It would protect the monster from Raigeki or Dark Hole. Basically any card effect that would destroy it, banish it, change its battle position, target it, prevent it from attacking, change its ATK or DEF or just do anything to the monster would not effect that monster.

What it doesn't do is prevent effects that don't do anything to the monster because the monster would not have been affected anyway and since Dimension Wall doesn't affect either monster, my opponent took the battle damage. It also won't prevent effects that affect your opponents monster(s), so the monster with this effect would not be able to destroy a monster that is equipped with Heart of Clear Water or Mist Body because it wouldn't be affected normally.

This guy was 740 or something ranking on DN so it goes to show that even those of us with vast knowledge of the game sometimes get caught out.