Continuity mistake: During Stoick's funeral Hiccup first shoots an arrow towards the ship and then the other characters draw their bows. Gobber is not shown as he is further away. In the next shot where they shoot their arrows, Valka and Eret are suddenly further apart from each other and Gobber has moved closer in between them. Eight arrows are shot simultaneously, but only seven arrows rain down.
Continuity mistake: When Hiccup has blasted Drago off his Alpha, he looks at the tail rising and sighs "Not again." If you look closely, his sword Inferno is both in his left hand and on his hip.
Continuity mistake: In the film Hiccup has a scar on his face which is made out to have been there since he was a little boy. But if you watch the first movie, he only has a faint mark in some scenes, such as the scene after when he was looking out the blacksmith window gazing at Astrid.
Continuity mistake: So during the whole dancing/singing scene between Stoick, Valka, Hiccup, and Gobber there we see that Gobber has a fake arm on the left and a fake leg on the right, but at the end of the movie we see it is the other way around.
Continuity mistake: When Hiccup uses the blindfold to prevent the control of Drago's Bewilderbeast, there is a tear on the blindfold, which is to the left of Toothless' head. In the next shot where it zooms in on Toothless's snout, it has moved slightly to the right. (01:23:20)
Chosen answer: That's kind of left in the middle, so that Hiccup can have his revenge without having to flat-out murder someone.
Friso94