Item does not exist The page you selected contains an item that does not exist. It may have been deleted by another user

When you do the migration from MOSS 2007 to sharepoint 2010, and if you use spcalendarview control in your custom webpart, it gives an error saying "Item does not exist The page you selected contains an item that does not exist. It may have been deleted by another user" when you click on an created event in the calendar, this is because of the text "&ID=" appended with the DisplayFormURL mentioned for the calendar property.To get rid of this issue, just add the property for Spcalendarview "EnableV4Rendering="false"



This has to resolve the issue.

For more details on this issue, please check the following link: http://social.msdn.microsoft.com/Forums/en/sharepointdevelopment/thread/e0b5d8f8-3c35-4ede-8686-c26234e847bd