MS update screw-up
Despite the title, MS is (surprisingly) not directly implicated here.
I spent a significant time working as a consultant for a major European company (name withheld to protect the innocent, not that I think they are). One time about 15 years or so ago, everyone got an e-mail from the IT department telling us to leave our desktop PCs switched on over night (you normally had to switch them off, or else) so that IT could push a major update out. All we would need to do is reboot the machines the next day to finalise the update.
Next day dawns and in I go, nice and early, with a pile of work that needs to be done on a PDQ basis. Reboot desktop, and all hell breaks loose - Windows fails to come up and instead gives me a whole slew of error messages. Facing a definite "pot of petunias" moment I take a deep breath and call the hell desk; after only waiting 10 minutes they pick up the phone and (surprisingly) say more-or-less immediately that an IT technician will be around sometime that morning.
When the techie turned up (after only 2 hours wait) he diagnosed the problem and fixed with with a surprising amount of speed. Feeling a little suspicious I asked him what happened, and he responded that the update pushed out the last night had failed. He then said the immortal words: "we (as in the IT crowd) are feeling pretty happy: the update only failed for half of the users; our testing suggested that three-quarters would be impacted".
This was on a site with something like 8,000 employees!