<add key="aspnet:MaxHttpCollectionKeys" value="1001" />
A security update of Microsoft aspnet breaks pages that contain more then 1000 post items. If you have pages likes this, the web.config key will fix it. Then again, maybe better to change the page?
Microsoft recently (12-29-2011) released an update to address several serious security vulnerabilities in the .NET Framework. One of the fixes introduced by MS11-100 temporarily mitigates a potential DoS attack involving hash table collisions. It appears this fix breaks pages that contain a lot of POST data.
If your application stashes page information into ASP.NET ViewState, and exceeds the web server threshold, you're going to run into this problem. Rather than applying the web.config fix problem straight away you might want to take a look at optimizing your code first.
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