The CLOCK_EXTENSION feature is a newly added feature that extends the remaining time left of a team to 3 hours if they have less than 3 hours left on the clock. 
It is possible for an attacker to always extend a game by creating a long chain of games in the dispute game and making a move below the 3 hour duration in order to extend it; although, they will likely lose their bonds for doing so. This is bound by the MAX_GAME_DEPTH which is 73 on mainnet, and therefore, the maximum time the game can be extended by will be 73 * 3 hours = 219 hours = ~9 days.
Although this is intentional as acknowledged by the NatSpec, it is still a possible attack and so that is why we filed it here.
