Years ago when working on a metal lathe, I was taking a heavy cut in 400 series stainless with good chip breakage, chips flying lazily, leaving little smoke trails. I was surprised when my shirt pocket started to get warm, then warmer than that as a fire started in my pocket. Fortunately it didn’t get too far, but it was certainly exciting.
I’ve had close calls but never suffered the injury yet, but a ball of glowing welding or cutting slag burning through the top of a boot has got to be one of the worst. (Or falling in a boot not tied up).
Always wear cotton around hot stuff, and be mindful of the power of showers of sparks.