Server Application Unavailable

This was my first ASP.Net 4.0 application deployment on IIS 6.0. The website was published under "Default Web Site" that hosts other Virtual Directories too, targeting ASP.Net 1.1 & 2.0.

When I tried to access the site, I got the error:

Server Application Unavailable

The web application you are attempting to access on this web server is currently unavailable. Please hit the "Refresh" button in your web browser to retry your request.

Administrator Note: An error message detailing the cause of this specific request failure can be found in the application event log of the web server. Please review this log entry to discover what caused this error to occur.

And the event viewer said:

It is not possible to run two different versions of ASP.NET in the same IIS process. Please use the IIS Administration Tool to reconfigure your server to run the application in a separate process.

Looking for resolution, I came across this blog: http://weblogs.asp.net/owscott/archive/2006/01/26/436607.aspx

I rechecked and my application was using the application pool created for .Net 4.0 apps; so the issue was not with application pool. After a bit of more wandering I started checking all websites that were using the .Net 4.0 application pool. And it turned out somebody had their website use the .Net 4.0 application pool but target .Net framework 2.0. Argh!!