Corrected entry: When Hiccup draws Toothless in the glen for the first time, he smudges the left tail section, indicating his understanding of the problem of why Toothless can't fly. When we next see the drawing, at dinner that night, the left tail section is clear and unsmudged. (And later in the film, it's smudged again.).
Moviegamer
3rd Apr 2010
Correction: This has been fixed. The tail IS smudged at the dinner scene.Apparently it was fixed whilst still in theaters.
Moviegamer