A senior QHI had one at Middle Wallop in a Lynx a few years ago, caused by a tiny hole at the 90 degree bend of a rigid fuel line on top of the starboard engine. He was on an instructional sortie with a basic student on finals to a field at night on goggles! The No2 fire warning illuminated and the student looked up to see a glow coming from behind his head - as the QHI landed very expeditiously, the No 1 fire warning came on as well! The fireball was sucked up and out by the eductor action, crossed over the firewall and set off the firewire on the good engine. Double engine fire indications in the dark in the middle of nowhere - no thanks!
Fortunately they landed safely and shut down very quickly - an engineering inspection showed that the aluminium trans decking had started to melt, trashing the elctrical bay underneath and severely threatening the integrity of the tail rotor driveshaft.
The hole in the fuel line had apparently been caused by a knackered generator bearing settting up a sympathetic vibration in the fuel line which wasn't of a very good design anyway.