Pressing alt freezes players and makes them invulnerable.

If you press alt ingame your client will freeze, the client will still receive packets, but it won't do anything with it, rendering the player invulnerable until they re-enter their screen or press alt again.

A video showing this:
http://www.youtube.com/watch?v=5SeZ--8tRt0

for as far as I know this bug has been in minecraft since even before Alpha, in that time almost everyone knew of it, but now people are exploiting it again.

I can vaguely remember it had something to do with LWJGL, the server used to kick players for this, but it doesn't anymore.