I rarely ask questions on Stackoverflow these days. The website is just so hostile to people asking anything, and any moderately advanced topic gets shot down as well with claims of “you shouldn’t do this” and “have you profiled your application to find out if this is a real issue”.
Monica was a moderator on 6 different sites and was a highly decorated moderator. She had contributed frequently to the growth of the entire Stack Exchange community and was appointed as a moderator on MSE in recognition of that. She was fired after she requested clarifications on a new policy and a change in the CoC. Here is her version of the story: Stack Overflow Inc., sinat chinam, and the goat for Azazel .
A lot of moderators then resigned due to the way Stack Exchange treated her. Firing mods and forced relicensing: is Stack Exchange still interested in cooperating with the community?. Stack Exchange later spoke about her in newspapers. The Director and CTO of Stack Exchange later posted public apologies, but didn't do anything to follow up their apologies. Finally after taking the legal route (Stack Overflow is doing me ongoing harm; it's time to fix it!), Monica was able to sign off on an agreement (Update: an agreement with Monica Cellio), which wasn't that helpful.
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You'll still see usernames that reference Monica, usually on meta SE.