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.
Before Toothless brings Hiccup and Astrid to the dragon's nest, we get several shots of other dragons. Pay attention to one shot of a Monstrous Nightmare; it can be seen holding Gloria, the hippo from Madagascar. (Confirmed on the DVD's commentary.).