Alex Bosworth provides a list for developers because “Ajax is also a dangerous technology for web developers, its power introduces a huge amount of UI problems as well as server side state problems and server load problems.”
1. Not giving immediate visual cues for clicking widgets.
2. Breaking the back button
3. Changing state with links (GET requests)
4. Blinking and changing parts of the page unexpectedly
5. Not using links I can pass to friends or bookmark
6. Too much code makes the browser slow
7. Inventing new UI conventions
8. Not cascading local changes to other parts of the page
9. Asynchronously performing batch operations
10. Scrolling the page and making me lose my place
Alex has a second post with more Ajax mistakes.