Way back in the mid '70s when lithium batteries first came along, Larry Penberthy (founder of MSR) sold a single D cell to replace the 4 D cells in a Justrite headlight. At the time, we all used the Justrite for mountain rescue because it had 10-24 hours of usable light, depending on the bulb.
It was specified that we had to have an air vent in the battery case because of the venting gas from the lithium cell. The battery case Penberthy sold was a plastic cigarette case with a cheap battery clamp to hold 2 cells, one was a shunt dummy to allow the single cell continuity. The batteries were very expensive for the day and only came as 3v D size.
Batteries have come a very long was since then and nowadays you do not see much about potential hazards mentioned, personally, I thought the original problems had been solved to the point venting was not so much an issue. I always wonder how my waterproof Inova with two CR123 batteries is holding up with the sealed case. So far, so good.
I once put a set of Energizer Titanium batteries in a Mini-Mag and while using it it got so hot I could not hold it in my hand, had to drop it and took it outside, turned off to let it cool. Later I checked on it, it was still very hot and after opening the light, one battery had expanded and leaked causing some sort of chemical reaction which caused the heating. The batteries were brand new and it would seem, did not like the sealed mini-mag case! I have not bought any more titanium batteries since and went back to standard alkalines.
Battery fires and explosions are not something most people expect, but certainly need to be aware of! Cheers!