Why do Ctrl+F2 and Ctrl+F3 sometimes stop working? 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?
I think I have figured this out:

ctrl+F1 apparently turns keyboard access on/off, which disables the other shortcuts. So I have now disabled this shortcut and hopefully the others will no longer get accidentally disabled.

If this happens to you, try ctrl+F1 to re-enable. Also you can disable this in:

 System Preferences Keyboard Keyboard &amp; Text Input