How to intentionally mess up a Mac? [closed]

Inspired by this question: How to mess up a PC running Windows 7?, I'm interested in repairable ways that you could intentionally 'mess up' a Mac computer.

Note from a Mod on the other question, that should also be mentioned here:

If you suggest ways to disable/break the computer, please include information on how to fix whatever it is you're breaking as well, if it is not immediately obvious.

One-liner answers, that fail to explain how they work or what the do, will be converted to comments.

Use Control-Option-Command-8 to invert the screen colors.

Press it again to turn it back, it's somewhere in the options so those that don't know the hotkey have to hunt.

Install MacDefender

If you can enable the root user, the easiest thing to do would be to chown the user folders of every user account, and then use chmod to prevent the original users accessing them. This is reversed easily enough, as the owner should be clear from the name of the user folder.

Add an AppleScript as a log in item with the following content:

tell application "System Events"
    log out
end tell

Then, every time the user logs in, they will be logged out again. Optionally add a (possibly random) delay and/or a humorous/obnoxious message. To get back use of the computer, hold the shift key down while logging in to disable log in items, then remove the log in item.

For a bit of nostalgia, remember doing similar things with a shut down AppleScript and the Startup Items folder on Mac OS?