The ones that gets me is when they drop a card that goes completely against their deck design. I once lost to a guy playing zoo who played a twilight Drake as a 4/2. Kept waiting for handlock cards but nope just a zoo deck with a card designed for keeping cards in your hand lol.
It's the 16-20 rank weirdness. People who know how to play but have to use crappy filler cards because they don't have a collection, or people with golden heroes playing random crap because they don't know any better.
Stabilized and got a win streak to get myself out of there. Just throws you when they play something unexpected that would otherwise be bad. It was a Midrange Hunter that obviously didn't have Call of the Wild.