A few years ago, I was running an oatmeal stout mash on my relatively new (to me) RIMS system when I received an important phone call. My cell coverage in my garage is terrible, so I stepped inside for "just a moment" to take the call.
Unfortunately, "just a moment" turned into perhaps 15-20 minutes. After ending the call, I headed back to the garage. As I was approaching the garage door from the kitchen, I could smell the distinct odor of burnt toast. "Hmm, that's weird.", I thought...
I opened the garage door and was greeted by the sight of a smoky haze filling the garage. Big greasy-looking bubbles of white smoke were erupting from my mash tun. Apparently shortly after I left the garage, my mash stuck. Without any wort flowing past the temp sensor, my PID controller kicked on the burner, and left it on. So the wort at the bottom of the tun boiled, then started to scorch, then burn. It was probably burning away in there for a good 10 minutes or more. Obviously the batch was ruined, and the bottom of my mash tun had a nice layer of baked-on carbon where the burner jets were. That carbon layer resisted all my feeble attempts to remove it using mere soap, PBW, etc. In the end only an overnight soak with over cleaner removed it.
So, obviously I nobody to blame but myself for walking away from a process involving flames. I'm just thankful I didn't set my garage on fire!