If you've recently lost a fight against another player who defended with a 100,000% defense boost, this means that you've just been Ambushed!

The 100,000% boost is a backend number designed to make sure Ambushes hurt even the strongest opponents! In many cases, a low-level player getting a 100% boost in defense still wouldn't be powerful enough to stave off an attack from a high-level player.

You'll only ever see this boost when you trigger an Ambush that was set against you.
To read more about Ambushes/Counter Attacks, check out this FAQ article on the subject.