Revealing mistake: When Flynn and Maximus are fighting to get the satchel, it flies off and gets snagged on a tree branch. But you can see it goes through the branch, and not around it.
Suggested correction: If you look closely, it is very clear that it doesn't go through the branch at all, but hangs from the side as it should.
Revealing mistake: In the beginning scene when the dragons are attacking Berk, it shows several sky torch things being raised into the air. As one closest to the camera is rising, a Viking throws a burning stick at it to ignite it. If you watch it closely, you can see the torch starts burning first to the far left, well out of reach of the thrown fire. Even allowing for fumes igniting, the fumes nearest the flame should light first.