As I was reading an article about How to Avoid the common pitfalls of Ajax code and found common Five Ajax Anti-patterns(An anti-pattern is an application design flaw that's seen often enough to be an issue everyone should watch out for), those are:
1. Polling on a timer when you don't need to
2. Not inspecting the return results in the callback
3. Passing complex XML when HTML would be better
4. Passing XML when you should pass JavaScript code
5. Doing too much on the server
The article is about why it's wrong and how to fix it :Ajax and XML: Five Ajax anti-patterns