Not exactly a new issue but an empty ASP.NET ImageUrl tripped us up today. In this case there were no visible symptoms but two requests were coming through when Fiddlering the page view: the first initiated by the user and the second initiated by the page itself towards the end of the trace. In development with a debugger attached, this was manifest with Page_Load, CreateChildControls, etc being called twice for no obvious reason.
Because I initially thought I introduced the problem with the control I was working on, I first attempted to convince ASP.NET CreateChildControls was complete; I did so by clearing the Controls collection before unleashing my own code and setting the ChildControlsCreated property true once done. Neither of these tricks had any affect and the Fiddler trace had me convinced something beyond the ASP.NET pipeline and IIS had to be at fault.
That turned out to be the case but ASP.NET was still to blame ;-)
Specifically, we were adding a server-side ASP.NET image control but not initialising its ImageUrl property; the image src was later being set by jQuery at runtime on the client side. As mentioned, there were no visible symptoms of the double request: the image src was set as expected by jQuery and everything appeared okay on the surface. The IE dev toolbar also showed image src as being correctly set and there were no 404s in the mix.
Notably this problem wasn't reproducible in Firefox or Chrome.
To fix the problem, we first set a default value for the ImageUrl property but that left me feeling dirty since it was still resulting in an unnecessary request. When I realised the server-side Image tag wasn't actually being used for anything server-side anyway, I replaced it with a boring old HTML img tag with no src. Microsoft has other, equally lame workarounds for this if you're interested; note they also don't plan to fix this bug.