I've had that happen on an old cheap offset. It had quite a bit of grease/oil build up from bacon grease to salmon fat drippings. Once I left the cooking chamber lid open while I brought some finished food inside. As I stepped back onto my deck a gust of wind charged the lump I was burning and a hot ember landed inthe cooking chamber. The fire started small, I was a bit intrigued by it so i watched it grow, and grow. I watched a little too long because it got too hot to approach and put the lid down. It lasted about 10 min then burnt itself out. The scary part was when I realized the burning oil dripping into the oil catch can that was on fire. I grabbed the fire extinguisher right away, but didn't need to use it. The good news was the cooking chamber was now clean.