I was pressing and dragging the mouse with Shift held down in some program that requires that.  I heard a beep, so I thought "oh, one of the keyboard a11y features kicked in".  But since I wasn't typing, I didn't care to see what had gotten changed.

Later, I couldn't type at all.  I couldn't even switch virtual terminals or kill the X server.

It turns out that:

1. slow-keys had been enabled by leaving Shift pressed
2. The confirmation dialog for "do you want to enable slow keys" popped up *behind* all my other windows, so I never actually knew what happened.
3. There was no indication anywhere that slow-keys was active.

Before actually being able to fix that, I tried ssh'ing into my machine remotely to try to see if there was a rogue process with a keyboard grab or something; a rather stressful situation :)

So, a few things:

* The confirmation window for a11y features should not be a normal window, because window managers *will* screw it up.  It has to be an override-redirect window, always on top.

* There has to be an indication somewhere when a11y features are enabled.

* Could slow-keys be *not* enabled when leaving shift held down, if a mouse button is also being pressed?  This would at least fix things for drawing programs that have you press down Shift for long periods of time while you mouse around.