First, please take a look in this video:
The baby is having fun, but imagine someone would cause the lights in your own room to turn on and off over and over and over again? Irritating, isn't it?
Now see this:
It's real. (Thanks Manishearth for helping me on this!)
Any user with upvote privilege can simply click the upvote button repeatedly to cause this to any other user, at any time.
How can we block this? I couldn't come with a concrete suggestion so your ideas are welcome.