Thanks for the response, and just to clarify,

This topic doesn't have to be about an entire film one doesn't like, but just the worst artistic mistake in a film. One mistake, however, doesn't mean one dislikes the whole film. For example, I greatly enjoyed the period film "Hugo" 2011, but I thought the opening title sequence in which the letters of the title just flashed onscreen in a font which seemed sort of futuristic was, to me, an artistic mistake, especially when today, we live in an era where beautiful and imaginative title designs are not just a possibility but are almost commonplace.

I feel like the entirety of the Last Airbender movie was the worst thing I've ever seen in cinema.  Almost everything about it was terrible.  I feel as if there was no decision made during the planning or production of the film which involved any sort of rational thought.  There are so many problems, the casting, the script, the cinematography, and such disrespect to the source material.

The only part of the film which is any good is the effects, which look remarkably lifelike.  It's almost as if some bad filmmakers just happened across some real benders.

But, other than that, I feel like the worst mistakes in movies are usually the more persistent ones, ones which you see in multiple movies, and several times in that same film.

One thing that really bugs me is editing errors.  For instance, two characters will be speaking, and the speaking character can be seen from the back of the head, however, the motion of their head and jaw do not quite match up to what you are hearing, as the sound and the visuals here are both from slightly separate takes.  I understand that the editor wants you to see the other actor's face and how they are reacting to the speaker's words, but I just can't stand such an obvious mistake right there on screen.
This happens in all sorts of films, although I remember noting it especially in Iron Man 2.  Even some of my most favourite film have this mistake.  Although, because of the nature of animated films, I don't think that they ever have this issue.

Another thing is overused stock sound effects.  In particular this specific horse vocalization, which can be heard in a great many films and TV shows, and oft times more than once in the same production.  I love Mulan, but I think that they use the same horse sound about seven times.  It also pops up in Pan's Labyrinth, and almost any movie featuring a horse.

There's a baby equivalent of the Wilhelm scream. I always refer to it as the '20th Century Fox' baby sound effect, because the 2 films I remember hearing it in first were Robots and Ice Age 3. Since then, I've noticed this overused sound effect, and I can't help but find it quite annoying.

I've seen trailers for some films like Trolls, which looks like a terrible movie (but then I guess it's not targeted at my age range), but other than that I have enjoyed every film that I have ever seen.

In terms of particular scenes from movies, here is a list:

Note: I could go on for a while about the CGI mistakes in the Star Wars prequels, so I will forget about those just this once

The opening of Star Wars: Rogue One was a bit disappointing for me, as you could easily tell that they used a model for Krennic's shuttle, and tried to use force perspective.

The Force Awakens had several scenes which strayed too close to the OT, like the locations, characters etc.

Lots of Marvel/DC films seem to have Superheroes who can survive 1000 punches, kicks, falls and still have the energy to run a marathon, which can sometimes take away the joy of watching a film.

There are also aspects of the film industry in general which I dislike, such as the tendency for characters to come out of water completely dry, a phone case changing colour between shots, handheld accessories appearing in the opposite hand etc.

About the Wilhelm scream, I don't really mind it being used in films (yes it is probably overused), although I have noticed that I can pinpoint the point of time in any of my favourite movies where there is a Wilhelm scream, so I have either seen them too many times or it is just used too much!

Jack & Jill. I can't believe I allowed my parents to pick which movie to watch it was awful.

Lucy. The film lost all dramatic tension when the main character became fundamentally unchallenged by the world around her. The film bases it's premise on the overused "Humans only use 10% of the brain" cliche and hypothesizes that once that percentage is breached, we basically become gods.