I recently got woken up due to a production web application I am responsible for being down. Most of the reports indicated the users were not getting .net errors, it simply was not responding to their requests for data.
When I logged onto the site, and navigated to the problem area, I was greated with this error: "A potentially dangerous Request.Form value was detected from the client"; along with the recommendation to disable EnableEventValidation if the error were incorrect.
In an effort to just get the site back up and running I flipped the switch on that page variable. I'm not sure why the clients never saw the error, but as soon as I flipped that variable I started seeing the same problem that they were reporting; no errors, just no response to requests for data.
Since my development server was functioning perfectly I had to use Response.Write 's on the production server to start tracking down what was going on. I was shocked to learn that the Page.IsPostBack variable was always returning true. I got the site functioning again by finding an alternative variable I could use in its place.
With the problem under control efforts were redirected to figuring out what triggered my bad morning.
My co-worker discovered that windows updates had been applied by our hosting company to our production server. Through the good old fashioned method of uninstalling until things started working again, he discovered that KB2894842 and KB2894843 were the culprits. Apparently Microsoft had found a security flaw in pages that had EnableViewStateMac turned off and the fix had resulted in some rather odd results for our site.
The compromise I finally settled on was adding a button with a PostBackUrl to my master page and hiding it with CSS.
I am still not sure why the Page.IsPostBack variable stopped functioning correctly and that is part of what prompted this post. Hopefully if I, or anyone else, runs into a similar problem in the future there will be a place to start looking due to this.