The Web application at (URL) could not be found

I received the following error : The Web application at (URL) could not be found. Verify that you have typed the URL correctly. If the URL should be serving existing content, the system administrator may need to add a new request URL mapping to the intended application while trying to access sharepoint site collection using sp api from an asp.net web application.

This was happening because the ASP.net application pool identity did not have enough previlages. Changing asp.net application pool to run under same identity as MOSS application fixed the issue.

More details can be found at Dragon Panjkov's blog.
http://www.sharepointblogs.com/panjkov/archive/2007/10/18/asp-net-moss-and-app-pool-identities.aspx

The security validation for this page is invalid

Error:

I received the following error while uploading files to a sharepoint document library from an asp.net application.

The security validation for this page is invalid. Click Back in your Web
browser, refresh the page, and try your operation again

Resolution:

Add following lines of code before the Update() method on SPFolder is called.

site.AllowUnsafeUpdates = true;
web.AllowUnsafeUpdates = true;

Multiple Copies of Sharepoint Web application

To create multiple copies of a web application on a webfarm the database backup-restore will not work as the GUID of site will be same. The following process is useful in that scenario:

• Back up using stsadm stsadm -o backup -url <original site url> -filename <backup file path>\db.bak
• Create new web application (and content database) using Central Admin
• Restore (and overwrite) using stsadm stsadm -o restore -url <new site url> -filename <backup file path>\db.bak -overwrite