How Strange

I have several DNN sites of various vintages - all older than 3.7 and younger than 6.2
Over the weekend I suddenly started getting error BC30560 failures.
This was affected many sites and different controls but all had the BC30560 in common.

Message: DotNetNuke.Services.Exceptions.ModuleLoadException: D:\laishley.eu\wwwroot\admin\Host\hostsettings.ascx(114): error BC30560: 'controls_skincontrol_ascx' is ambiguous in the namespace 'ASP'. ---> System.Web.HttpCompileException:

My hosting provider assures me nothing had changed and so I started to look for old dll's.

I ended up fixing the first site with a core upgrade but then had the same sorts of error on a clean 4.5 site..
I tried stuff like restarting with a webconfig kick but no luck.

There are a few posts on DNN on this issue number but most seem to relate to specific modules.

A goggle search found the post linked below and this seems to work.
Posted here in case anyone else is experiencing the same strange problem.
I would really like to pin down a cause for this - it makes me very jumpy when things like this happen
Anyway:
http://www.netdruids.com/Blogs/tabid/54/EntryID/4/Default.aspx

Where Mad Druid says:

• set to true the debug mode in web.config: <COMPILATION debug="true" strict="false">
• restart the application connecting to the home page
• re-set to false the debug mode in web.config: <COMPILATION debug="false" strict="false">

What fun.