I used the give command to give myself a double tall flower with damage value 6 and it broke my single player map. It crashed and I can't get back in. I used an external editor to change the double tall flower to damage value 1 and that didn't fix it (it still tries to render it as value 6 according to the crash report). I also changed the block id of it to 1 (stone) and that didn't fix it. Ended up reverting to a backup I made yesterday (I always backup a map before using my map editor on it for the first time that day...thank god!). I haven't lost almost anything but I had visions of people intentionally doing this on people's multiplayer servers to nuke worlds (hence the private security level on this message, but for all I know it could be single player only (I doubt it)).