Did you know that every time an Ajax control performs a partial postpack, every event in the life cycle of the control's containing page or pages fires?
To me, this seems counterintuitive - There is no refresh of the containing page or of the master page, yet the Page_Load of both events fire.
I ran into it when I witnessed some unexpected behavior in a colleague's Ajax control. After some investigation I saw the behavior was caused by code in the Page_Load event handler. I thought this was a bug until I learned it was by design. So we ended up bracketing some of the Page_Load code, testing the value of Page.IsPostBack to prevent code from running when it should not.