I know it's been discussed here several times, but I think I have some conclusive evidence as to the cause of the stone 8.

Typically when this comes up we argue about whether the ball deflects too much and sends the 5 in front of the 8 or whether the ball does not deflect enough and drives the 5 straight back between the 8 and the 9.

I believe that the first scenario does happen, but usually these are easy to see.  Balls that either hit very light or very flat leave this type 8 pin and this is often what happens on the 8 - 10 leave.

I don't believe that the second scenario mentioned above ever happens unless either the 5 or 8 are off spot.  If the ball gets enough of the 5 to send it to the right of the 8 the ball will take out the 8 itself.

After studying the 8 pin that WRW left on the show Sunday, I now know what happens.  The ball enters the pocket and continues on a path that would normally drive the 5 into the 8 just as it should, but the head pin is what causes the problem.  The head pin rebounds off the 2 into the 5 and drives the 5 to the right of the 8.  The ball never touches the 5 or the 8.

The only true tap in bowling?  I think so.