Help with Backfire

Anything that can put more stress on the pocket in the rocker:
- Stiff springs and high lifts
- Stuck valves (for a wide variety of reasons; I had some old bad gas at start up of an engine and an intake valve stuck due the gum in the old gas and did in a rocker)
- Over revving and valve float (lifters pump out; valetrain overextended)
- Old worn rocker pockets
- Defective rockers