Double spend

– a phenomena where a bad actor attempts to spend the same coin(s) twice, in order to fraudulently extract double the value of their cryptocurrency holdings.  

Double-spends may occur when the receiver of the coins credits the user for a purchase before receiving the necessary blockchain confirmations, which would invalidate one of the double spend transactions.

It's the main way to 'overtake' Bitcoin..

I'm sure people attempt double spends daily on the Bitcoin network, but the network is designed to fight back on this.