I brewed a Stone IPA clone last night and had a stove top partial mash of 2lbs pale 2row and 14oz of Crystal 20. I got my water up to 165 and then put the grain in, but the water dropped too low... down to 148. I turned the gas range on in short bursts to get it up to 154 for the mash. After the temp was in range I had to go upstairs and put my 2 year old to bed. When I came back down my temp was 168 and climbing! I couldn't figure out what in the world was happening.
I dumped a couple cups of cold water in the mash and stirred it up, but I feared the enzymes were already destroyed. The temp was up there for well over 5 five minutes. Come to find out after staring in disbelief, the cast-iron grate over the gas burner was holding a TON of heat and heating the pot up after I killed the flame. I probably got 10-15 minutes of stable temperature before it took off into the 160s. Luckily it was a fairly small portion of my grain bill and I still have 6lbs DME to ferment out, but now I'm afraid the beer is going to be too sweet and FG won't come down. Time will tell, I guess.
Lesson learned, if you're going to stove top mash it's best to remove the pot from the stove completely and put it on a hot-pad instead.