Well, I don't watch a lot of movies, but there's one that comes up as an immediate example. It's already mentioned above... Yep.
Avatar - The Last Airbender (M Night Shyamalan)
There were so many things dear ole' Shyamalan got wrong right from the get go. And the source material's already so well fleshed out too. I suppose he wanted to get creative or something... But seriously, for a movie with audience appeal stemming largely from an established fanbase of the original cartoon (or some might argue anime)... He shouldn't have disrespected the series in the way that he did. From mispronouncing the character's names, to ruining some character personalities (Sokka is the case in point); he was already setting himself up for a massive fall.
That, alongside his horrendous choices of child actors, excessive narration, over-usage of slow-motion... Also, that blasted 3D bullshit, as it turns out, 3D was barely even used! That said, the CGI for some scenes were ridiculous as well. A vivid example I recall is the earth benders breaking out of their imprisonment. In the original, because they were
EARTH BENDERS, they were imprisoned on some ship away from any source of earth. Due to that, they were unable to resist or break out of their captivity. In the movie, for crying out loud, they were pretty much kept in a village-like place, I think? Goodness gracious, they had an abundance of earth to use but they didn't. And when Aang rallied (which itself is a change; the original had Katara rallying the prisoners) the prisoners, what did they do to fight against their captors? The bended earth, yes, but WTF MAN
WTF IS THIS SHIT. THERE'S LIKE 6 OF THEM AND ALL THEY MANAGED TO BEND WAS WHAT?
The earth benders in the original series could dig tunnels and lift much heavier boulders... And all it took was just one!
The laughter in that theatre though. Sigh.
So yes. Put very mildly, this movie was a colossal fuck up. I don't think any other movies will come close to this to me though, largely due to the fact I really liked the original series. Then someone just had to go in and shit all over it. THE PAIN. IT'S REAL!