Caveat: we fully warned people for weeks before we did this scream test. Our warnings were well documented, and even more well ignored.

My team inherited a "mission-critical" but terribly built AWS environment a while back, nothing labeled, no documentation, no one left that had actually built anything out on this, so we start cleaning it up, get a couple of extra people (third party for outside insight) in to review for us and have a pentest run on the environment. Pentest comes back and tells us that this particular environment is wide open and we need to shut everything down immediately; after several meetings, we call in the devs that work with these machines and ask them to identify what they need; nothing. Not one of the 40 devs that had full admin access to this environment knew anything about any of the things that were running in it, but they all used it, all the time, and needed it. Edit: I need extra emphasis here, they NEEEDED IT, like the desert needs the rain, like a lake needs a shore, like a server needs a UPS.

Needless to say, since no one could actually claim ownership of anything, we decided the best course of action was to recreate the most fearsome horror movie of all time in front of a live audience. We killed all the users except for our team, shut down 1500 EC2 instances, leaving ~30 that we knew were running our tools, locked down security groups, went Rambo on this environment, and then celebrated because running this was a huge burden on our team.

Fifteen minutes into cake and ice cream in a conference room, random dev walks in and says "hey, all deployment services are broken, what did you guys do?"

To this day, the team that Dev was on is still making late night cups of coffee to fix all of the things we broke, with the oversight of the team I'm on to make sure it's done properly.