How to fix Invalid viewstate and Validation of viewstate MAC failed in asp.net?

There are couple errors in asp.net related to viewstate
1. System.Web.HttpException: The client disconnected. ---> System.Web.UI.ViewStateException: Invalid viewstate.

2. 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. ---> System.Web.UI.ViewStateException: Invalid viewstate.

How to fix?
1. Check Application Pool Recycling IIS setting
The fix in this case is to adjust the settings on the application pools so that recycling is less likely to occur at peak periods.
2. put machineKey section into your web.config (http://www.aspnetresources.com/tools/keycreator.aspx)
Such as:
<machineKey
validationKey="80FE3D40B4CDD7FBC1DC32FA0DC87C83586B651214A19F4BC140970DEAE7F99A6B24D2323DB2088E472194BF14CB996062B41F9C2D3A4D821091AA53282F55D5"
decryptionKey="DA085C28238653E0C85994B40AB4208236E0E814BF698F9C202A80901C00F613"
validation="SHA1"
decryption="AES"
/>

References:
1. Intermittent Invalid Viewstate Error in ASP.NET Web pages
http://support.microsoft.com/default.aspx?scid=kb;en-us;555353&sd=rss&spid=6351
2.ASP.NET state management: viewstate
http://www.aspnetresources.com/articles/ViewState.aspx#

How to handle 404 error in ASP.NET

1. Add following section in web.config
<customErrors mode="On">
<error statusCode="404" redirect="~/error404.aspx" />
</customErrors>

2. Get wrong page from query string
string from = Request.QueryString["aspxerrorpath"];

http://rayaspnet.blogspot.ca/2012/08/handling-exceptions-and-404-errors-in.html

Validators and Javascript in asp.net

ASP.NET Validation in Depth
http://msdn.microsoft.com/en-us/library/aa479045.aspx

How to detect session tiomeout

http://aspalliance.com/520_Detecting_ASPNET_Session_Timeouts.2

How to fix missed Navigate Backward and Forward buttons in Visual Studio

In VS Click Tools,
Import and Export Settings,
select Import Selected environment settings and click Next,
select Yes, save my current settings and click Next,
select General Development Settings, and click Finish.


Or use shortcut: Ctrl+- and Ctrl Shift +-
http://blogs.msdn.com/b/zainnab/archive/2010/03/01/navigate-backward-and-navigate-forward-vstipedit0024.aspx