openlayers/openlayers — comment on commit e3cc96dbfb58ee873db457e74de7c7774a25bf45
File: lib/OpenLayers/BaseTypes/Class.js line 94
By tschaub:

Unfortunately, this breaks things in IE where we send event instances to the `extend` method. The check below (about `sourceIsEvent`) would need to be performed above and used here as well. I'll add an alternative suggestion on the original pull request.