Compromise any Windows XP machine (that you have physical access to) with one single line of code. Even if you're logged on as guest, this cmd line text will upgrade your account to root level on the fly, after which time you can do anything you wish to the machine, (even reformat the drive & install linux!). ACHTUNG: Link goes to video that, for inexplicable reasons, has Limp Bizkit for the soundtrack.
News 14 in North Carolina had an online system where people could post school closings, etc that would scroll on the screen. The postings all got reviewed before they went live, but after they’d been reviewed/approved, the system was open for the original poster to go back and change the copy, so you could essentially submit something benign, then go back and monkey with it. Screen shots of the results.