This does not happen all the time; about once in a few days.

After killing the engine, sometimes it does not die immediately - the engine would shake and shudder first, before coming to a complete stop.

If I attempt to start the engine immediately after that, it would be quite difficult - i suspect it's due to "overflow" because I can smell petrol if I open the hood. Waiting for some time (20-30 minutes) would solve this "difficult to start" problem.

What's going on, and what would...