Q:

Viewstate MAC failed error in ASP.NET using Autogenerate key, not on farm or cluster

Error is:
"Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that  configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster."

I am not hosting on a Web Farm or cluster.
Application has about 100 users.
Only 1 user getting this message.
User only getting this error message in IE11, not when using other browsers (Chrome, Firefox) which leads me to believe this is browser related.
Running IE11 in no-addons mode same error.
Autogenerate machine key defaults in IIS are untouched.

My goal is to figure out what is causing this, and not just a quick fix.
If anyone has had this same situation happen to them and can point me in the right direction it would be appreciated.

A:

I could not figure out what was causing this issue, but I managed to fix it.
In IE11 developer tools (F12), emulation tab, I switched the Document Mode from Edge to IE10, and then back to Edge. No joke, this fixed the issue. 
Sometimes just turning things off and then back on fixes the problem. If anyone knows why this could clear something up additional comments are welcome. Thanks.

