About 15 or 16 years ago an AS355 operating in the police role suffered an engine fire caption at night while returning to the base airport. The pilot followed the emergency checklist which included shutting down the affected engine and operating the first fire bottle. This was all done, "i.a.w".
The fire caption did not go out so the second bottle was used. The fire warning light still did not go out. The crew also experienced smoke and fumes in the cockpit so the pilot elected to carry out an OEI landing short of the airport, in a field.
Thankfully, no damage was done to the airframe or its occupants. It proved to be a false engine fire warning but the smoke and fumes were real enough! They came about because of an unusual and misunderstood item in the RFM/checklist, which was a translation from French. The note stated that the fire extinguisher buttons should be "unlatched" after being operated. There was no further explanation given. I was flying the type at the time this occurred (my first job flying it) and I had previously asked our trainers what this meant, but no meaningful explanation was forthcoming.
What actually happens when the FIRE button is pressed is that a squib operates electrically, via a control circuit board, to release a sealing cap which normally holds back the extinguishant. The fire button/switch (a "STACO" type), is not of the perhaps more widely used type. Most others are a simple spring loaded button which pops out again when released.
However, these particular fire bottle buttons are held in the "IN/energised" position by an internal latching mechanism. The button has to be pressed again to release the latch.
Now, as the squib fires, the small filament inside is designed to burn through, giving an open circuit once it's done its job. However, experience showed that in some cases, the squib fired but the element did not burn through. If this occurred, as it did in this case, the circuit board would overheat, giving smoke and fumes in the cockpit (it's by the pilot's left knee, in the centre console)!
So, a spurious engine fire warning can result in the crew inducing a real, but small, cockpit fire. Just what you want, especially at night..
After the investigation, the CAA recommended that Eurocopter undertook to replace the fire bottle switches with a non-latching type (straight swap and presumably not expensive). They refused, saying that the incident was a false warning! The CAA did not follow this up so those buttons are probably still out there.