I use keyboard shortcuts a lot, one of my favourites is ctrl+F2 (access the menu-bar with the keyboard).
However, sometimes this just stops working. Whenever this problem happens, ctrl+F3 (access the Dock using the keyboard) also stops working. Pressing ctrl+F2 jumps the selection to the bottom in Finder instead of opening the menu.
This happens in Mountain Lion and I remember that this also used to happen in Lion and Snow Leopard. I suspect it's something weird but deliberate, like accidentally performing an unknown shortcut that changes a setting somewhere.
What causes this, how can I avoid it, and how do I rectify it when it happens?