The first use of a new security product my manager insisted we roll out (as a duplicate to an existing tool from another group) was to quarantine a change in a system file that seemed to be spreading through all of the PCs.
Except the change was to quarantine explorer.exe which was being changed with a patch that just got pushed out. The net result was about 6 hours of the desktop group wondering "why the hell are all of the PCs not logging in right after this patch" followed by about a month of rolling tickets from seldom used computers that had just been powered off.
His excuse was it only showed a file hash in the main screen and you had to view details to see the name plus he had a 3 day change open to roll out the system. Never understood how he got away with that one but such things did catch up to him about 2 years later.
To all machines. Employee and servers alike.
Yes. Including the DNS servers.
Took them a day or two to work out how to roll that one back.