yes i happens sometimes, so i dont remember how did i solved it, btw i am using ff2 so here is what i do to fix this fkng problem:
first i refresh/reload (but as much as i remember it doesnt helps), so then i logout close MD tab in my browser i clear cache, restart browser and login again, after all this it must work normal (i think that it is even enough to logout and login, but not sure)