cache from shooting themselves in the foot: This doesn't seem to happen often,
but it has happened in internal code. I'm going to add more debugging info to
#440 as we get it, but want to get this in to prevent people from shooting
themselves in the foot in IE accidentally.
git-svn-id: http://svn.openlayers.org/trunk/openlayers@2024 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf