As said above, to hot a plug can detonate. When close to the edge, as in slow traffic on a hot day and shut the motor off you can get run on where a piece of carbon can glow and cause the engine to run on acting like a glow plug. I remember when I was an apprentice airplane mechanic in the 50s, A private plane pulled up to the pump for gas, plane wes the predecessor to the harvard, mostly plywood with an inverted 6 cyl Ranger inline aircooled engine, some of the old farts can jog my memory as to what it was. Anyway, when they shut the mags off the engine slowed down and almost quit, then reved up like you turned the mag back on, but not so. When the back cyls got hot as it slowed down, theplugs would start glowing and rev the engine back up. When it got a blast of cold air when the engine reved up the back cyls would cool off and shut off again. This would go on as long as it had fuel. the only way to shut it off was to shut the fuel off.