Happened to me ONCE...second attempt it got away and shot to over 500*.
You can't leave 'em alone while you are starting up, especially with the vent open at all.
Remember, I am just a backyard jack, but I learned from LA and "experience" that the best way to start it is to raise that temp is very slowly for the first few cooks till you get a handle on it.
Somebody will jump in here to give some details. Believe me, it'll be alright.