Routing mishap with upcut spiral bit; what did I do wrong?

I just bought a 1/4" spiral upcutting bit from Grizzly's clearance site.
I was using it on a router table to lengthen the slot on a 1/2" thick featherboard. I had the bit set so it was just barely above the top of the wood. Since it is supposed to have a 5/8" cut, that should be okay, and it looked okay. I had the featherboard against a fence, intending to cut one side and then flip it over to do the full slot.
However, it was just barely cutting, so I was about to take it out and put in a straight bit, when it grabbed the featherboard, bounced it around for a second and then spit it out at a high speed, cutting a slot clean through it as it went.
No harm was done, except to the trashed featherboard, but it was a little unnerving.
Any ideas what I did wrong? I would guess that the cut wasn't 5/8" as claimed, but it looks to be that long. Or is this simply why it was on clearance? I bought the bit to cut groves as part of tongue and grove joints because I thought it would clear chips better than the straight bit, but am a bit leery of that now
A related question; I had have always heard that you should set a bit so it is not quite in all the way. On a straight bit like this, I had trouble doing that. Any tricks? Thanks.
Interestingly, my spell checker objects to "router". You would think that it would be in there for a network router at least.
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First, never trust your spellchecker. It missed "grooves". I'm thinking you might have asked too much of the bit by having it "just over the top" of your workpiece, rather than about half-depth. It grabbed the workpiece 'tween the fence and the bit. I think... Glad you're okay. Tom > I had the featherboard against a fence, intending to cut one

Someday, it'll all be over....
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Suspect you lost some control as you lost the center and did a little climb cutting. Climb cutting on the router table, knowing or unwittingly will be met with surprise. I would not mess with the sprial cutter without more than average experience. They (fully) engage the work like no other cutter, they want to pick up the work or pull the cutter from the collet. Just too much risk without substantial experience. http://www.patwarner.com (Routers) ***********************************************

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