Engine shutoff oddity

[K. Roman]

Sometimes not often, I'd say once every month or so, I shut off my engine after a drive and the engine will sort of sputter, or chug-chug for about a second before it completely shuts down.
I feel it may be the way I sometimes turn the key. It's usually when I'm listening to a cd and I'll turn the key a little slower.
Bad habit don't know why I do it.
Anyone know what this is???

[ken_xman]

Check your timing. It could be dieseling..... compression burning fuel. Only usual if timing is really far advanced.

[svandamme]

could be carbon build up, the build up get's hot , glows... and ignites the fuel and keeps the engine running