The other day, my neighbor came over and asked me if I could route a 2 3/4" circular hole in some cedar, but not go all the way thru. He wanted it to "sit" on top of a post (for a mailbox, I think). I told him no problem, that I would have it ready for him the next day.
I proceeded to make a template, that I then attached double sided tape to, and attached said template to the piece of cedar. Then mounted this in my Router table, hoping to use it with the template, to make a very nice circle.
I then flipped the piece over, routing blind, and started to take out 1/8" increments using a 1/4" spiral cutting bit. On the third time, as I was routing, the router made some sort of clatter, and then a kind of a shrill sound. I immediately turned off the router, and waited for it to stop. When I lifted the piece off, I found that the top flange of the insert that held the guide bushing had become separated, and the guide bushing was just spinning around the router bit.
I learned 2 lessons from this that I will try to never do again:
1) Never route blind again. Take the time to set up the plunge router, and do it where I can see what is going on.
2) Never use hard board on anything that might have any sort of stresses put on it. I think the sideways pressure really contributed to the hardboard separating.
I am very lucky that this didn't turn into something more.