My favourite: we paid a consultancy a fortune to convert an app that we calculated was saving the company a minimum of £100 million a year (in the 80s when £100 mill was big money).
When we came to test the code, we saw some very strange results. I investigated and found that in the crucial part of the code, a couple of hundred lines of the original had been converted to a single comment that read: "I couldn't understand this so I left it out".
And that part of the conversion had been done by .. the principal consultant.
Another consultant, when asked why his code wasn't commented and looked like he'd had a fit at the keyboard, said in perfect seriousness "It was hard to write, it should be hard to understand".
He might have been the guy who gave an urgent fix I'd written a critical failure (= absolute no go) for .. a typo in a comment.
Another time, at a trading company, an operator chatting to a mate in the server room, leant on the emergency power cut off and killed all the servers in the building. There was a pause of maybe 15 seconds before every phone in IT lit up with irate traders screaming about their screens going blank.