Just for the record, I'm aware that lithium batteries have exploded. I've been flying R/C planes for longer than seriously riding bicycles and that's the hobby that has really had the brunt of it (and rightfully so, as we tend to really abuse our batteries.)

However, your "There is a tread just about that. Something like that happend to one German cyclist" made me think that the thread was on bikeforums, but I couldn't find it (because I made an incorrect assumption.) As for exploding/burning battery threads, there isn't just one -- there's dozens, perhaps hundreds. Check out rcgroups some time.

Though I've seen more than a few pictures of burned planes, cars and houses -- I've never seen one that really hurt anybody, so that's a new one.

If you're worried about buildup of pressure, drilling a hole or two in your flashlight would give it somewhere to escape. And yes, this can happen with non-lithium batteries too.

The danger is going to be less with single cell setups. And as the thread said -- "we don't often get reports of exploding 18650" -- but yes, it obviously can happen.