"OMG -- there's a hacker on PvP!!!11111oneoneonetwothree!"
It's called de-sync, kiddies. 
In all fairness, most of the time when you're new to a multiplayer online game and you see something really screwy, it's not outlandish to think someone is doing something they shouldn't be.
QFT. This was the first game I had ever experienced desync problems of this kind, so the first time I saw it, I actually got really mad and called the guy a hacker.
I consider big mistakes to be careless gameplay mistakes that fail to capitalize on flaws/holes or fail to maximize quantum use. This also includes counting cards to determine the probability of drawing specific cards needed to win a game.
