Please consider removing the AutoPostBack = true declaration from RadListBox should posting back to the server is not desired. The set_cancel property in the SelectedIndexChanging event args serves for canceling the selection of the given item, not for preventing a postback. However, it indeed makes sense that canceling the selectionIndexChanging should not postback the page. We will research what side effects might such change have to RadListBox.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items