Can anyone tell me what it means when nock hits forward edge of the riser?

I would geuss that your nock travel is not level.

Sorry Redneck Bowhunter not the shelf but the side of the riser (the forward edge). It hits hard enough to acutally break a prong of the nock off of one. It doesn't happen on all of the arrows either.

I had the same problem, and it turned out to be a bad nock point. One arrow would hit while the rest got off without a hitch. When I took a close look at the nock, it turned out that I had almost split it (Robin Hood) a group or two before and it was spread out a good bit with a mark from where it had been hit.

My bad. In your second post I was thinking limb twist before you mentioned that it does not happen with every arrow.

I'd have to say there is a spine issue. Or perhaps the nocks are to tight. What is your setup?

or knocks not aligned properly

I'm at 51 lbs @27" with a 29" 500 spine with a 150 gr tip. It makes sense that if the nock was too tight the string rolling off my fingers would send it towards the riser wouldn't it?

Hmmm.... It doesn't happen on each shot, huh? I suggest next time this happens to mark the arrow that does it. Check to see if it's the same arrow each time.

Otherwise, I'd say your arrows are seriously underspined. Or your brace height is too low. Or your nocks fit too tight. Low brace and nocks that are too tight will definitely interfere with paradox and overspine arrows always contact the belly side of the riser.