The first prints of this movie showed Hiccup's drawing of a Night Fury with its tail intact, when the drawing was dropped onto the Dragon Book. This was a continuity error, since part of the tail fin on that drawing had been erased in a previous scene. Many digital theaters had a corrected version of the movie, showing that erased tail, by mid April 2010. (The DVD and Blu-Ray discs also have the corrected tail at the Dragon Book scene.)
When Hiccup first attached the artificial tail fin, Toothless is distracted by eating fish from a basket. Toothless takes off with Hiccup on his tail, we get a shot of the ground - the basket is missing.
When Toothless pins Hiccup to the rock after being set free, we see trees in the reflections of his eyes. Toothless was looking straight down at Hiccup, where no trees could possibly have been seen.
When Hiccup is searching for the dragon he shot down, he hits a tree branch which flings back in his face. In the next shot Hiccup hasn't moved, but the branch is gone.
During the dragon training scene with the zippleback, Tuffnut is dragged into the gas cloud by the dragon and loses his hat, leaving it in plain view. He then runs out of the cloud wearing the hat again.
When Astrid follows Hiccup into the valley, she has an axe, when Toothless attacks, Hiccup then has the axe and tosses it away. Next scene the axe is gone.
Astrid's patronym is Hofferson when, according to Norse traditions, it should be Hoffersdóttir. While patronymic names are no longer used exclusively in Nordic countries, and while the suffixes have since changed (-dotter/-datter in Sweden, Norway, and Denmark; though Iceland still uses -dóttir), during the Viking era patronyms were always used in place of family names and -dóttir was the suffix for a daughter.
In the training arena, when Toothless prepares to shoot a plasma blast, a green gas is seen forming in his mouth. In all other media this gas is either purple or blue.
The Vikings' accent is Scottish, not Scandinavian. However, the Vikings settled large portions of Scotland, including the Inner Hebrides where the fictitious colony of Berk is shown to be located, and this is the movie's way of reminding us of this.
It took a while for Hiccup to learn how to fly with Toothless. He had to design the tail/control mechanism that would allow him to control the tail. Without Hiccup's control, Toothless would crash. When Toothless "kidnaps" Astrid he flies totally out of control, skimming the water, doing fast ascensions, barrel rolls, etc. Because Hiccup and Toothless are so out of sync, they should have crashed almost immediately. However, it is established that Toothless is able to fly for a few seconds even without Hiccup. Evidently, even if Hiccup is not prepared for sudden maneuvers, he can adapt quickly and apply proper tail position at a moment's notice (as he's demonstrated earlier in the movie). He has no choice but to play along or they'd crash.
The Vikings use many words such as "port and starboard" which did not exist at the time. We are hearing a translation of what they would have been saying.
All the children have American accents, while all the adults have Scottish ones. This is clearly a deliberate choice by the filmmakers, meant, in part, to emphasize the difference between adults, who are stuck in their ways, and children, who are open to the new possibilities. Anyway, since the characters aren't "really" speaking English (we, supposedly, are hearing a "translation"), their accents are artificial constructs, not representative of how they "truly" sound.
The actual size of Toothless varies from one scene to another (compared to Hiccup's height).
After the final battle when Hiccup gets out of bed, his feet swing to the floor from the wrong direction. His pillows are on the right side of the screen so his feet should come from the left side.
Toothless is stated to weigh 1,776 pounds. After jumping onto the crowd of people gathered around Hiccup at the end of the movie, he lands on Spitelout's shoulder. Despite Toothless's massive weight, Spitelout manages to stay standing and is only thrown somewhat off balance. Likewise, when Toothless is in the arena with Stoick, Stoick most likely would have been crushed from Toothless being on top of him, yet is perfectly fine and doesn't show any sign of injury.
After Astrid apologizes to Hiccup, Toothless's tail fin can briefly be seen on the wrong side.
After Hiccup wakes up in his village and sees that his foot is gone. Toothless is jumping around the house. When he jumps on the floor again, his left side of the tail, which should be prosthetic, is intact.