lots of times that happens to me, and it's because of some fine bits of flint that fall out of the end of my lighter and into the bowl... it's too small to see, dust really, but when the fire hits it - it sparks. freaks me out because I think it's a seed that might blow up sending hot embers into my eye.

And dude I love it when food/drink tastes like something else when I'm stoned!