Employing Ghislaine Maxwell has to top the list. Changing their community rules definition of hate speech to only be against chosen groups has to count too. But there’s a lot of bad stuff that redditors quietly accepted, forgot, or newer members never heard of.
A mod picking a fight over a days old point mid conversation with someone else, while not logged in as a mod, logging in, and permanently banning a user. That’s how I learned there is no effective mod oversight.