My friend Oli has a lens which is quite old and it has gone a bit mouldy unfortunately (Yes, lenses get mouldy like bread does if left for too long in the dark) He left it with the front lens cap off in his room, hoping to get some sun into the lens to kill off the mould to stop it getting worse . However it was a rare afternoon in Durham which the sun was blazing! The sunlight was pretty much shining straight into the lens and just like Physics back at school, a lens + light = FIRE! According to Oli, "smoke was pouring out" and it burnt through the rear lens cap! (see photo above)
If you dont use your kit often, it is recommended to get your kit and lenses out from storage every now and then to avoid mould and other problems (or to clean them, or the guilty pleasure of admiring their glow in the light *blush*), be VERY CAREFUL if you leave them in the sun though. They can probably set fire to most things if the light was right and they were left for too long, as my friend Oli found out the hard way.
Photo credits: Oliver Grimmett, check out his work here - www.olivergrimmett.co.uk