This morning I installed the Telerik editor Sharepoint web part on my dev server justfine, but when I went to the client site, it produced a Parser Error that seemed to be coming from the Master Page.

When I looked in the Event Viewer, there was the following error:

Safe mode did not start successfully. Could not load file or assembly ‘System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35’ or one of its dependencies. The system cannot find the file specified.

The solution was found here: http://www.telerik.com/community/forums/thread/b311D-bktbdd.aspx

In my case, the AJAX controls weren’t even installed to begin with. I installed v 1.0 following the instructions provided here;http://sharepoint.microsoft.com/blogs/mike/Lists/Posts/Post.aspx?ID=3 and that cleared things up.