I love messing around with commands, but unfortunately I did it on my favorite world. I wanted to make it so that any mob (excluding passive ones, villagers, and iron golems) that walked within 10 blocks of me when I had my armor on was subject to the /kill command. I, unfortunately, set keepInventory to on and the command block to repeat. So, when it failed and killed ALL entities within a 10 block radius of me, I got killed too since I was within 10 blocks of me. But, since keepInventory was on, the INSTANT I respawned I died again. That went on for a very long time with me trying to access the chat right before I died again to try to turn keepInventory off. It failed and I ended up having to delete the world.
Moral of the story: ALWAYS test command blocks on worlds you couldn't care less about so you don't have to end up deleting your favorite world.