I've observed that after using the shift-cmd-4 key combination used in OS X to take a screenshot, that the shift key release state is not propagated in glfw. That is, my glfwSetKeyCallback doesn't fire, and calls to glfwGetKey() show the shift key as still pressed.
I've added the following workaround locally within cocoa_window.m: http://pastie.org/7729967. In my application, I'm only relying on glfwGetKey() for this behavior so this works fine, but I haven't investigated further to determine the appropriate strategy for dispatching this change as an event.
I'm happy to prepare a patch to address this if that's helpful, otherwise just a heads up :)