Recently I was routing a decorative edge on a large round tabletop. About halfway around, my router started to vibrate and I noticed (with great dismay) that the depth of the routed pattern had become much deeper than when I started. I turned off the router and when I lifted it off the table the router bit fell out and hit the floor. After uttering a few choice words, I called a friend for help. He diagnosed that I had installed my router bit too far into the collet. He said that the shank of some router bits flares out just under the cutting head and if you tighten the collet against this flared area it’s likely to come loose during use.
Sure enough, the shank of my router bit was in fact flared below the cutting head. Never wanting to experience such a costly mistake again, I added a short section of plastic tubing to each of my router bits to prevent them from going too deep into the collet. I found the tubing in the plumbing section of the hardware store for 50 cents per foot.