I generally dislike these Top 10 something list sorts of posts, but I still wrote one - sigh. This is about UX mistakes that have been identified, analyzed and criticised by UX experts, yet somehow still linger. Feel free to use it as support in arguments with project managers.
Carousels They are misused all the time, because they provide a (fake) way to put a lot of important things on top of a page. Which is way easier than admitting you have a limited amount of space and user attention. I don't want to jump on the hater bandwagon, but in most circumstances they are a terrible choice. If you must, reduce the number of items to 3-4 and configure the carousel not to progress automatically.
Opening external links in new windows People from marketing once heard this technique prevents visitors from leaving your page. I prefer to believe most people can master the use of tabs and back button. Smashing magazine and Jakob Nielsen say we shouldn't take control from users. Trust them.
Complicating when it's not necessary It would be too plain without it is a terrible argument. Make the culprit some coffee and talk him into trying a one column layout first.
Obscuring links for looks Many designers find underlined links ugly and unacceptable. Don't let them go too far with this and remind them to use different colors for :visited links. Here are some great guidelines.
Have some extremely useful references: