Ozymandias in Watchmen
Watchmen did a pretty good job at staying true to the original series in terms of costume design, except for one unfortunate character… Ozymandias. The directors decided to go from the purple cloth and jewelry of the original design and move towards a more "modern" superhero movie look. For some reason, that means that the character's nipples were put into the design. Weird.
Captain America in The Avengers
The Avengers introduced some changes to Captain America's costume. The colors were way brighter and the mask was part of the costume instead of a separate piece. A lot of fans felt like that made the costume cheesy and cheap-looking. The designers wanted to go more "traditional" but it didn't really work. Fans were not pleased, but are they ever?