When a 'repeat' command block has '/kill @e' in the box and you turn it on, you can no longer do anything because as soon as you respawn you die again.

Steps to reproduce:
1) Place a blank command block on repeat mode.
2) type '/kill @e' (or any other commands you can die with, this is just the one I used)
3) Power on the command block with a constant power source.
4) You die infinately.