A glass paperweight, a wax-soaked cardboard fruit box, and sun at 10am on an autumn morning - I noticed smoke and took pics (this was ~14 years ago, but re-found the pic today)
It caught alight right after I took the photo. easy to put out with fingers, but damn... if I hadn't been there....
@NanoRaptor I had almost exactly this experience with a kid's plastic snowglobe left on a bed by a window that only gets sun later in the day. I smelled the smoke, picked up the globe before anything caught fire... but "if I hadn't been there..." doth resonate.