I've been using a 400 watt hps bulb on a 1000 watt hps ballast for about a week now. The bulb didn't immediately blow. I then put a 150 watt hps bulb on the same 1000 watt ballast. It produces much whiter light than when running on the 150 watt ballast, but it still didn't "blow up". After about an hour I removed the 150 watt bulb with no apparent damage.
Added response: Please do not try this at home guys- the last thing you want is a house fire. Using the wrong size bulb in an HID fixture is just plain dangerous.
