Reinstate Monica:

Why are there a lot of users with Reinstate Monica usernames

https://meta.stackexchange.com/questions/340868/why-are-there-a-lot-of-users-with-reinstate-monica-in-the-username

Monica's explanation of what happened

https://judaism.meta.stackexchange.com/questions/5193/stack-overflow-inc-sinat-chinam-and-the-goat-for-azazel