I have started a fire, under ideal conditions, by concentrating sunlight through the convex bottom of a glass bottle.
It was far from easy, and seems unlikely to occur accidently, but if you wait long long enough, and discard enough broken bottles, then the improbable will eventually occur.

I heard somwhere that most forest or wilderness fires are started by lightning.
The risks of a fire being started immediatly by a lightning strike are of course well known.
Perhaps less well known, is that a large tree struck by lightning may smoulder internaly for MONTHS before conditions become suitable for rapid spread.
Whilst the tree remains standing and the surroundings damp, spread is unlikely, but when the smouldering tree falls onto wind dried leaves etc, then spread is likely.

In areas at especial risk, fire watchers atop towers or in aircraft, search for smoke.
They sometimes find a tree smouldering, weeks or even months after the last thunderstorm.