It's 2016 and Sitefinity is up to version 8.2 and the problem still exists.
The problem is typically experienced when a widget template is dependent upon a skin file that Sitefinity no longer supports.
For example, I just upgraded from version 6.3 to 8.2. I had some custom calendar templates that were based on one of the built in calendar templates from 6.3. When I say "based on" I mean I literally copy and pasted the code from a built in calendar template and then made my modifications.
The built in templates in version 6.3 had controls that looked like this:
<telerik:RadWindowManager id="radWindowManager" runat="server" Modal="True" Skin="Sitefinity">
The built in templates in version 8.2 changed the name of the skin file:
<telerik:RadWindowManager id="radWindowManager" runat="server" Modal="True" Skin="Default">
Needless to say, when I ran the site after upgrading to 8.2 the calendar widgets were crashing because they couldn't find the "Sitefinity" skin file and displaying the error reference in the topic. I can only imagine that someone thought it would be "better" to change the name of the default skin to "Default" instead of "Sitefinity", but then neglected to comprehend the negative impact this change would have.
It wasn't a huge cause of frustration as I was able to locate and fix the problem rather quickly, but as someone else above already pointed out, this is not very professional.