When the twins are putting up their sign, Tuffnut lets go of his side while talking, forcing Ruffnut to hold the sign alone except when the shot pans out, she is holding Tuffnut's side instead of hers.
After snotlout closes the door to the cell there is clearly no lock or chain on it , but in the very next shot there is a chain and lock.
Hiccup is thrown in the cell and you can see him looking through the bars , however a few minutes later when snotlout is throwing himself in the dungeon the door to the cell is clearly standing wide open .
The twins declare the letter s to be illegal , yet they use it several times in just about every sentence and every other word.
Astrid is still seen outside the dungeon when fishlegs is brought in for failing to amuse ruff and tuffnut , and when snotlout puts them in the cell , you hear him put the chain on and then walk out without securing the lock .