Getting Strange Error " Invalid viewstate."

I just check system log , and got this strange error, Does anyone know what's causing this error ?



Log type:      Unknown
Severity:      11
Message:      Invalid viewstate.
Exception:      System.Web.HttpException: Invalid viewstate. at System.Web.UI.Page.DecryptStringWithIV(String s, IVType ivType) at System.Web.UI.Page.DecryptString(String s) at System.Web.Handlers.ScriptResourceHandler.DecryptParameter(NameValueCollection queryString) at System.Web.Handlers.ScriptResourceHandler.ProcessRequestInternal(HttpResponse response, NameValueCollection queryString, VirtualFileReader fileReader) at System.Web.Handlers.ScriptResourceHandler.ProcessRequest(HttpContext context) at System.Web.Handlers.ScriptResourceHandler.System.Web.IHttpHandler.ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

if you search the forum, you will find some posts recently about viewstate problems - there are links with more information - sorry i can't remember exactly where these posts were!

I did search the whole forum, but there is no specific posted solution for this problem here, few people were discussing about the same issue but they were discussing about why they are getting this error, but no solution on the forum

Ok i found a link posted by Andrei,

http://www.developmentnow.com/blog/InvalidViewstate+Or+Unable+To+Validate+Data+Error.aspx

This link  explains the cause of error but still i don't know how to avoid this error on website based on nopCommerce