Despite Hiccup insisting that on Berk it "snows nine months out of the year and hails the other three," neither snow nor hail are seen throughout the entire movie; however, this is pretty realistic for an unhappy teenager describing his or her hometown, even today.
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.
The actual size of Toothless varies from one scene to another (compared to Hiccup's height).
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.
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.)
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.
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.
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.
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.
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.
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.
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.
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. After Astrid finds Hiccup and Toothless, she runs off and they get her up a tree. Then Hiccup convinces her to get on Toothless so he can show how well he works with Toothless. At this point, Toothless flies totally out of control, skimming the water, fast ascensions, barrel rolls, etc. Because Hiccup and Toothless are so out of sync, they should have crashed almost immediately.
After Astrid apologizes to Hiccup, Toothless's tail fin can briefly be seen on the wrong side.
All of the children have American accents, the adults have Scottish ones.
At the beginning and end of the movie, Hiccup States that it snows during 9 months of the year and hails the other 3, but not once during the movie did it snow or hail, there was always sunlight.
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.