"Instant Karma's gonna get you / Gonna knock you right on the head / You better get yourself together / Pretty soon you're gonna be dead." So sang John Lennon, but fortunately there's no actual death here (well, if you discard social death) just some people getting dealt a blow by cosmic forces seeking to redress the balance.
Instant karma isn't just a Lennon song though, or just a subreddit, it's real and it's coming for you if you do something dumb or outrageous or you try to humiliate someone.
And there are some fine examples in this 'Instant Karma Fails Compilation', everything from people jumping a red light and getting their comeuppance to a scare prank gone ouch.
Be careful laughing too hard though, or karma may come for you. Because Karma's a biatch.
And it never forgets.