Was the bit 3/4" or was it smaller and you were on the 2nd pass? If it was the 2nd pass then feed direction was wrong. In that situation you have to make the inside cut first and then the outside cut. (standard right to left feed). If you reverse that then the inside cut is a climb cut.
I agree with the others that a jig and handheld is a better way to go. Someone will post their exact width dado jig sooner or later here. With it you use a smaller bit and make two passes, one along each side of the jig. Most finished ply isn't the size they say it is. It started out that way but then they sanded it and it winds up up undersize when that happens. The exact dado width jig solves that problem.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.