I lose the ability to input data from the keyboard after entering numerous hard returns in Microsoft Word or TeachText. I hear a beep, then the keyboard won't accept any more keystrokes, it just makes a strange beep. Even quitting the application doesn't fix it.

This doesn't happen with extensions off, but occurs even immediately after a clean install.

This is a feature of Easy Access called Slow Keys. Slow Keys gets turned on whenever the return key is held down for 8 consecutive seconds (there's a warning beep after 4 seconds). Thus, even though all features of the Easy Access control panel may be off, Slow Keys can still activate itself.

Slow Keys is supposed to prevent accidental key presses by waiting a brief amount of time before accepting a keystroke. This delay makes it appear as though the keyboard is locked up.

When this happens, turn off Slow Keys in the Easy Access control panel (the mouse will still be functional). Be aware, however, that holding the return key down again for another 8 seconds will turn it back on. The long-term solution is simply to disable or remove Easy Access.