That's not Lightspawn... you're thinking about Arcane Golem; a 4 atk/2 health with Charge whose downside is that it gives your opponent a mana crystal. Then he put several Power Overwhelming spells on it which boosts it stats for 1 mana (which downside is that it instantly dies the next turn, but that doesn't matter when he plans to win that very turn anyway).
Unless I'm mistaken(?) Only the Warlock class says "That's a mistake" (Priest says "My apologies"). What you described makes sense to me too though, minus the "charge". He must have had discounted spells and minions in his hand from a card like Emperor Taurissen.
There's no cards in Priest that give your minions charge. It says "0/5" for the stats in the case that it is silenced by something, those are the stats it's going to be.
Also, right-click on the opponent's hero and click "Squelch" and you don't have to hear them say "That was a mistake" the entire game.
edit: the only other possibility is that you're playing as Warrior and the turn before the enemy Priest used Nefarian to get spells from your class for the Charge spell