What would make a Chevy small block backfire?

Let's see, out of time, burned valve, stuck valve, valves adjusted too tight, stretched timing chain, flat cam, crossed plug wires. What is happening is this, the plug is firing the mixture in the cylinder, a valve is not sealing the explosion. Or, the spark plug is firing at the wrong time, thus igniting the "charge" in the exhaust, or intake manifold. In other Words, the valve, and ignition timing, (spark) are out of sync.