When Snotlout throws Hookfang the bread his neck spike are there, but in the next shot they're gone. Then his neck spikes are back when the camera shows him after Hiccup gives the others bread for the training. Then later when the riders catch their dragons eating the food Hookfang is missing his neck spikes yet again.
After Hiccup and Toothless' trick Toothless fires three plasma blasts, yet the sound of the boom and lighting show four.
When Bucket and Mulch get fish in the nets Bucket's arm brace is gray instead of brown.