While holding the inventory button, the screen rapidly flashes between the inventory menu and the regular view, as if the button is being pressed MANY times over, and that's not the only key that's being affected, merely the most noticeable, since EVERY key acts like this, but most of the effects are negligible, if apparent at all.

Sometimes simply traveling a few chunks from where you logged in fixes it, but most of the time, you can't do anything but press another key while still holding the first one to stop it from flickering on that single press.