Today at work, I suddenly saw some coils of smoke on other side of the double desk.
I thought it was coming in from the open window, so I went around to close it.
Then I saw that there was a very bright spot on the desk and realised what was happening. The co-worker who sits opposite me has bad eyesight and occasionally uses a desk-mounted illuminable magnifying glass. Said magnifying glass was positioned rather unfortunately, and sun + magnifying glass at the right distance + wooden desk = charred black hole on the desk and smoke - and who knows what else if I hadn't moved the glass away.
(As an aside, one could see the path of the sun; there was a small arc with occasional dark spots on the desk, though none as big as the one which had started smoldering.)