Generally speaking, you should never compare iterators from two different compilers

http://stackoverflow.com/questions/4657513/comparing-iterators-from-different-containers
It seems like this was just an overlook that happened to work on most systems. I was not able to thoroughly test this change, but it fixed an issue I was having in VC10.
It's possible the prev.end() was a copy paste error that was meant to be new.end() anyways.

apps/openmw/mwmechanics/magiceffects.cpp

@@ -151,8 +151,7 @@ namespace MWMechanics
         for (Collection::const_iterator iter (prev.begin()); iter!=prev.end(); ++iter)
         {
             Collection::const_iterator other = now.mCollection.find (iter->first);
-
-            if (other==prev.end())
+			if (other==now.end())
             {
                 result.add (iter->first, EffectParam() - iter->second);
             }