I just lost two of my three League combats to bugs.
In the first one, my third dragon defeated the enemy's third dragon. HP went to 0, enemy dragon disappears... and then I get hit with another attack. My dragon dies, I lose, even though I had already won.
Second one, I had just defeated the enemy's second dragon, and the whole thing hangs. Third dragon never shows up. I had to refresh the page, and the combat was gone. Grr.