Was it from a Bisharp, Crawdaunt, Conkeldurr or Scizor? They have ridiculously strong Knock Offs, and it hits 97.5 base power if holding an item before STAB, so even the tankiest of walls have trouble dealing with it. Bisharp's Knock Off gets about 143~ BP with STAB, and when coming off his strong 125 base attack and usually Life Orb, it hurts. Rotom-W isn't a wall per say, but a bulky support. Since Knock Off is from physical attackers, just throw a Will-O-Wisp on them and they'll be crippled for the rest of the game.