There is a bug when object.prototype has custom functions.

Actually it was detected when jquery was used with json.js (it has Object.prototype.toJSONString() function)

http://dev.jquery.com/browser/trunk/jquery/src/event/event.js#L164

164	        for ( var j in c ) {
165	            // Pass in a reference to the handler function itself
166	            // So that we can later remove it
167	            args[0].handler = c[j];
168	            args[0].data = c[j].data;
169	
170	            if ( c[j].apply( this, args ) === false ) {
171	                event.preventDefault();
172	                event.stopPropagation();
173	                returnValue = false;
174	            }
175	        }

should be

for ( var j in c ) {
   if (c.hasOwnProperty(j)) {
      ...
   }
}