This is a list I am compiling about every continuity error, out-of-character moment, major plot hole, and tonal consistency errors from HTTYD THW. This is meant NOT to be of personal opinion, and if anything of personal opinion is brought up, it will be edited into a non-biased way at a later point.
1#: "Night Furies can't survive in the cold", yet in HTTYD2 and Gift of the Night Fury, we see Toothless in the cold/snowy environments of winter-time Berk and the exterior of the Bewilderbeast nest and hes completely fine.
2# The Light Fury is first scene cloaked in a cage, but when she decloaks, it is shown she has a muzzle on. If she can't use her fire to cloak, and her cloak goes out fast, how did she cloak in the first place.
3# In the flashback scene, the knowledge of the Hidden World is described as a sailors myth, so obviously Hiccup and the Berkian's are NOT the only ones to know of its existence. And if people get smarter, which they seem to be, they will put together this piece of logic (Dragons went missing? Search the ancestral home of all Dragons FIRST then spread out) and easily find the Dragons...or at least where they ARE.
4# Even though it is described that the Hidden World is only accessible by flight, it is never shown that it is inaccessible by flight, making it less of a fact or more of a option/point-of-view.
5# Hiccup seems to have reverted back to his more vulnerable and less sure of himself/his abilities personality of HTTYD1, contradicting with his personality from HTTYD2.
6# Even though there is no good reason for the Dragons to go to the Hidden World (I mean, they lived through a 300 YEAR LONG WAR and didn't think about once going there? I mean, that's when they SHOULD have gone, and then what about all the time during RTTE or HTTYD2?) they still go and basically allow Grimmel to win (even though he's dead).
7# If Toothless never came back from the Hidden World, Grimmel's plan would be useless and he would have just lost not only his target, but also the Light Fury in the process. For a villain who is supposed to be extremely smart, this is a very risky stupid move that would NOT have worked if Hiccup and Astrid didn't go to the Hidden World.
8# The idea that the Dragons are restricted to Berk is false, as it has been shown before that the Dragons are not HELD or FORCED on Berk.
9# Toothless did not stay on Berk to help protect it (He does protect it, but that is not a reason), he stayed because he and Hiccup are best friends (As demonstrated in HTTYD1, 2, RTTE, and more).
10# The retcon that Toothless destroyed the auto-tail in GOTNF because he had no use for it just adds a plot hole, especially considering that if he got rid of it cause there was no reason for it...then what about when Hiccup was in danger multiple times during the TV shows, HTTYD2, and other? Times when the auto-tail would have made things easier. That, and the fact that Hiccup could live without the guilt that Toothless was 60-80% dependent on him for flight and (most likely) food and other things because of his mistake.
11# RTTE' canonicity is called into major question. On one hand, Hiccup's map has Dragon's Edge on it, while on the other hand the film ignores the fact that Dagur and Heather have their own Dragons who most likely wouldn't just go to the Hidden World out of the blue with no catalyst.
Please submit any of your own ideas for this list, and I'll edit it. And if you believe any or all of these points are wrong, please provide a fact based non-biased argument as to why WITH evidence from the movies/shows, and I will remove them from the list.