Stoick: When we crack this mountain, all hell is going to break loose.
Gobber: In my undies. Good thing I brought extras.
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.