The Error is Something Like this :
Unable to validate data.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Web.HttpException: Unable to validate data.
Source Error:
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
When the encrypt/decrypt key of view state change you got this error. somthimes it happen when user keep a page for a long while.
<configuration>
<system.web>
<machineKey validationKey='A96B2F3BF50A1A2A1B81214A550B89F2A318B3712FB5AC8CD82
AF4F3925B3C6ED58CF5F38503629A78BF8C27B1
B736C4E84DACE342AB83A42932B44EBC3D7601'
decryptionKey='D01201C69DF5E9863133028CBD4E6EEFC45B5E609D7F4107' validation='SHA1'/>
...
...
The default value for it is ...
<machineKey validationKey="AutoGenerate,IsolateApps"
decryptionKey="AutoGenerate,IsolateApps"
validation="SHA1"/>
http://www.experts-exchange.com/Programming/Programming_Languages/Dot_Net/ASP_DOT_NET/Q_21321364.html