Hiccup: I really did hit one!
Hiccup: He never listens!
Gobber: Runs in the family!
Hiccup: And when he does its always with this disappointed scowl, like some one skimmed on the meat in his sandwich. 'Excuse me barmaid, I'm afraid you brought me the wrong offspring! I ordered an extra large boy with beefy arms, extra guts and glory on the side. This here, this is a talking fish-bone!'
Gobber: Now, you're thinking about this all wrong! It's not so much what you look like, it's what's inside that he can't stand.
Hiccup: Thank you for summing that up!
Gobber: Look the point is, stop trying so hard to be something you're not!
When Astrid gets on Toothless for the first time, Toothless takes off uncontrollably and tries to scare her. Hiccup seems to have no control over him at this time. How is that so if with the new tail fin, Toothless cannot fly without Hiccup's precise control of the tail fin? That whole sequence would have fallen apart. This mistake was even admitted to on the DVD Commentary.
In the scene after Hiccup finally gets Toothless' new tail fin to work, and the two are relaxing after they fly through the fire ball, the two are greeted by a group of tiny dragons. This little dragon was actually what Toothless was originally going to be, as he was in the book, until they changed him to a dragon big enough for Hiccup to ride to suit the movie.