Straight to your questions:
Pawel, I am glad I could help you solve the problem. I cannot tell you exactly what causes this IE behavior because this will require some further research about browser specific behavior. What I assume is that there is some difference in the controls initialization and the time point this happens because the different browsers work with some small differences in the speed but this is only an assumption. However, even if that worked under IE without clearing the TargetControls collection, it is strongly recommended to do so because otherwise it gets populated with new and new controls, the rendered HTML will get bigger and bigger and this will influence the performance without a benefit.
Rok, there are known issues with MS AJAX framework and the Back button of the browser - note that this is related to the framework itself and not to RadControls, which are simply based on it. You can find more information about this on the net, e.g start from here:
Clive, I am glad that your problem was also solved, such cases are part of the reasons we need to examine reproduction demos to see what is exactly happening and this helps a lot to provide you with a faster and better support.
Guys, I hope that my assistance was helpful for you, in case you experience any furtehr problems, do not hesitate to contact us again, we will be glad to help!
the Telerik team