One area of user interface development that could stand some improvement is spell-checking. I’m not aware of any widely-used system for spell-checking a web application’s template or resource files, but good spelling certainly helps an app look more professional.
The latest spelling mistake in an application I’ve seen comes from the “Edit App” page for Facebook developers. One of the fields on the page lets you specify the “Canvas Page URL”, the URL within apps.facebook.com where your app can be found. It uses Ajax to give you instant feedback as to whether the URL is available or not as you type — but the word “Available” is misspelled: