Every time a new Hearthstone expansion is released it takes everyone a while to catch up to all of the new strategies that can be found within the various cards and their combinations. That goes for the pro players too, as one recent stream of top Arena player, Johnny "Ratsmah" Lee shows. He found himself suddenly decimated by a combination of his own cards' weaknesses and his opponents' hidden strengths.
In the above clip linked to by Kotaku, we see Ratsmah starting off with a very strong hold on the board. He has a trio of creatures who are all at full health, offering a combined 21 potential damage on the coming turn. His opponent needs to do something drastic to even hold him off, let alone win.
But that's exactly what they do.
Using a new Warlock card called Defile, which deals one damage to all minions and then retriggers if anything dies, Ratsmah's opponent plays the card which kills one of his own cards, and then another and another, continuing to do some damage to Ratsmah's own creatures, two of which happen to be Bittertide Hydras. When those take damage, they do three damage to their hero, so each time Defile was triggered, it did a further six damage to Ratsmah's hero, ultimately wiping him out in a single turn.
The look of shock on his face is priceless and perfectly counters his incredulity at the start of the clip when he says "yea, hit yourself in the face, that'll show me."
Funnily enough, it does.