The above error will be thrown at the time of updating Library or Lists items due to authentication problem, or We didn’t have the rights to update the items.
By using the follwoing code, We can overcome this issue,

Web.AllowUnsafeUpdates = true;
item["Title"]=”Sample”;
item.Update();

or we can also use the follwoing code to overcome the above error, this will affect the WebApplication. This will change the Security settings of a Web Application to allow the anonymous user to update item.

web.Site.WebApplication.FormDigestSettings.Enabled = false;
item["Title"]=”Sample”;
item.Update();
web.Site.WebApplication.FormDigestSettings.Enabled = true;

or we can change the settings in Central Administration to anonymously update the items. But this method will be dangerous.

• Central Administration > Application Management > Web Application General Settings
• Choose the WebApplication to overwrite the settings
• Select the option Off in Web Page Security Validation as follows,

 This was the error, i recieved recently when i open an Excel sheet in browser.  I didn’t recived this error before, so i searched in recent log file. That file list out the following line

03/04/2009 14:27:33.07    w3wp.exe (0×14C4)    0×020C     Excel Services      Excel Services Web Front End  5pad    Critical     No Excel Calculation Services is available in the farm.[User: SHAREPOINT\system]

Then i understood, there is no Excel Calculation Services available in my Server Farm. So, I moved to my Central Administration Page,

• Central Administration -> Operations -> Services on Server 
  
•  Select Excel Calculation server role radio button
  
• Start the Excel Calaculation Services

Now I moved back to my Excel sheets Library and clicked the Excel File. Now all works fine for me…