I wanted to update an ASP.Net application I maintain to the latest version of Ajax Control Toolkit, only to get an exception raised by the toolkit
Could not load control AjaxControlToolkit.[ControlName]. The script reference(s) of this control was not loaded correctly. If AjaxControlToolkit.config is used, probably this control is not registered properly.
See also posts on SO such as here and here.
The internets suggest not to use the latest version, as not versions are created equal, and point to the January 2013 edition as reliable.