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. I’ve compiled a list of the many mistakes developers using Ajax often make.
He read my mind on the item below — this is the the number one error people are making with Ajax:
How do you “break the back button.” Simple: when the back button does something other than what the user expects it to. And how do you know exactly what the user expects the back button to do? That’s what you get paid to predict, my friend.