When the firework explodes theres a light effect which was just added to the firework in 12w50a. Now in 1.4.6 Prerelease, when the firework bursts really close water, it creates a weird flash effect. The water sorta dissapears for a millasecond. To test the bug out, just make a puddle of water, then put a roof over it about 5 blocks above, then set a firework off so it hits the roof and doesnt shoot up in the sky, and you will see the water light effect happen when it bursts. Hopefully this can be fixed