I just solved a rather frustating problem I was having in an .aspx-page. All the events on the page had stopped working; at first I thought it was just a cause of 'rebinding' the events in InitializeComponent(). No such luck.
Go to plan B: I got the previous (working) version from SourceSafe. At that point my confidence dropped somewhat: it still didn't work.
Checking the HTML that was being generated by ASP.NET eventually led me to the solution. A couple of hours before the page stopped working I had accidentally deleted the Default Website from IIS. Although that was easy enough to solve, I didn't fix everything. IIS had 'forgotten' about the include that ASP.NET uses to do client-side validation.
After running aspnet_regiis everything worked as before.