As much as I'd love to just simply cop out and say "the entire internet". That isn't completely accurate (it's up there, don't get me wrong).
Here's just a few things I regret discovering instead:
- DeviantArt's dark community of horrifying fetishes. In fact, I might as well just extend it to every other image posting site.
- The existence of people like Deeker, Andrew Dobson, Nick Bate, Anita Sarkesian and so on.
- The hive of scum and idiocy that is Wizardchan. Also 4Chan's existance as a whole.
- Bronies. Just the bronies. What else can I say about them?
- The fact that everyone, whether or not they have a mental deficiency in real life, can and at times, will, act like a retarded moron on the internet.