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.
I have not been part of any major (to the point that it’s widely known and infamous) singular mod abuse, but I would say collectively, mods for some of the brand- or product-based subreddits downplaying or censoring valid criticisms and concerns about the product. I think it can be very insidious, especially in a discussion-based content aggregator system where you assume discussions are started from a grassroots perspective.