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.