When Astrid climbs on Stormfly's back after she tells everyone to get their dragons, Stormfly's head spikes are missing. But when the Whispering Death looks at Stormfly her head spikes are back.
When Hiccup goes to sleep he has his blanket, but when he wakes up the next morning his blanket is gone.
There is no way that Fishlegs is reading about the Whispering Death in the Book Of Dragons, because the page shows the Night Fury in the book.
When Hiccup goes to pull the Whispering Death spike out of Toothless' leg, the puncture is like a small hole in his leg with the spike in it then disappears after Hiccup pulls it out.