Yes I would expect surprises from spiral cutters, more on that:
At no.12 http://patwarner.com/faq.html
****************************
J. Clarke wrote:
Was cutting a 3/16" wide dado in a piece of Lyptus using an Amana
upcut spiral bit. Was cutting 3/16 deep in half inch stock. Router
is a Porter-Cable 7518 in a lift.

All was going well, when, after cutting four pieces, on the fifth one
I'm about three inches along and all of a sudden the bit comes right
through the top--it's still cutting just decided to pull out of the
end of the collet. So I put it back, socked it down hard, and
continued the cut and three inches along it did it _again_.

This time I pulled the bit and the collet and cleaned both with
lacquer thinner and got a lot of black crud off both, but I'm pretty
sure I didn't get it all out of the collet. Whether the problem is
fixed I don't know--I only had two short cuts after that.

If I had been holding the piece above the bit then it would have gone
right through my fingers. The notion that there's wood between you
and the bit is false security.