In the early 70s, Andy was doing a performance climb on an export Hunter at Dunsfold. That aircraft had a cabin altitude of about 22,000 ft when the aircraft was at 45,000 ft (a low level of pressurisation by today’s standards I know - but that is how it was then). By 30,000 ft he felt unwell so checked all his oxygen indications. All fine, both flow and pressure. By 40 he felt rotten and decided he was ill. He checked the oxygen system again but no problems showed up. He decided he needed to land asap before whatever was wrong with him got worse. He remembered putting the aircraft into a rapid descent but the next thing he recalled was waking up to find himself supersonic and going through 10,000 ft very quickly. He sorted things out and landed. When we checked his oxygen cylinders, they had been filled with air. Now that is what I call a gotcha. When he had selected 100% oxygen and emergency flow, it blew out nicely all round his mask so everything about the system seemed normal. In those circumstances what is a guy to think? Just imagine if he had tent-pegged. Would they have ever dug deep enough to find the bottles? Probably not and it would have been put down to just another unexplained pilot problem. In the event an accident was avoided because of the pilot's physical characteristics.
Now that is an interesting HF incident-reminds me of the time one of our aircrafts fuselages was deiced instead of the wings. Good attempt but 100% wrong!