Causes of Unable To Validate Data

I'm getting "Unable to validate data" in a webfarm scenario but not all the time. I'm getting it about on AVERAGE 1x or 2x per hour but fairly random. All of the causes of this error that I know of are list below:

1. Machinekey in the machine.config (or web.config) not set the same in a web farm scenario. Mine are set the same in the machine.config.

2. A page is submitted after a server recycle and the machinekey is set to auto. I'm using a static key which should eliminate this source.

3. A postback is triggered before a page is loaded. I'm using 3.5 SP1 with the render viewstate at top set to true.

4. A page that post back to another page or us开发者_如何学运维es server transfer. I'm not.

5. Serializing an anonymous type into the viewstate

6. Anti-virus programs or web accelerators on client machine.

7. A malicious attempt to modify viewstate

These are all of the sources that I know. Any other sources appreciated. Is this fairly typical to see this error 10x-20x times per day?

I see the same errors from time to time from a bunch of our 'heavier' internal applications. I have been at a user's desk when the error was created (logs show the time and user) and there was never actually an error presented to the users. I think it can happen when a user switches requests quickly without the page acutally loading and only partial data is transmitted to the server.

Does the page use ViewState heavily? Eg. controls that repeat like GridView, Repeater, ListView, etc...

It is very rare in the cases I have seen but you should try and verify if a user actually sees this error.

I have actually thought of putting a button on our custom error page in certain cases that says, 'click me if you see this error page' but have never actually implemented it.