HTTYD 1.
in the scene where Hiccup first finds Toothless tied up and tries to kill him he then stops and says i did this. then in the scene where toothless save Hiccup after they kill the queen dragon (Red Death) his father (Stoic) says i did this.
in the scene when they find the dragon island there is a tail up high on the rock that suddenly disappears.
HTTYD 2.
i noticed this after i thought about it for a while, so toothless lost his left tail fin, and hiccup lost his left foot, and my favorite hiccup is left handed. weird huh.
well that all i know so far!