Can anyone explain this Server Error to me?

I'm experienced with VS but new to ASP and web hosting. I have nopcommerce up and running, thanks to help on the forums. I was playing with it today and got this. Any guidance/insights appreciated. Thanks.

HTML:

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

Source Error: 


[No relevant source lines]
 

Source File: App_Web_mpe7ep5y.54.cs    Line: 0 

Stack Trace: 


[HttpException (0x80004005): Unable to validate data.]
   System.Web.Configuration.MachineKeySection.GetDecodedData(Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Int32& dataLength) +289
   System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +140

[ViewStateException: Invalid viewstate. 
	Client IP: 75.106.255.38
	Port: 59597
	User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0)
	ViewState: 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...]

[HttpException (0x80004005): Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.]
   System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError) +106
   System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState) +14
   System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +242
   System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) +4
   System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) +37
   System.Web.UI.HiddenFieldPageStatePersister.Load() +207
   System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +105
   System.Web.UI.Page.LoadAllState() +43
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6785
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +242
   System.Web.UI.Page.ProcessRequest() +80
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +21
   System.Web.UI.Page.ProcessRequest(HttpContext context) +49
   ASP.category_aspx.ProcessRequest(HttpContext context) in App_Web_mpe7ep5y.54.cs:0
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +181
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

 


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Version Information: Microsoft .NET Framework Version:2.0.50727.4200; ASP.NET Version:2.0.50727.4016 

 

Never mind. I can't reproduce the error. Works OK now. Something just hiccupped I guess. Dell

 

This error typically means that you lost your session and machine address key. It can be due to your application pool recycling.

 

That makes sense. I had done an app pool recycle a few minutes before the error occured. Thanks.