If your toilet rocked at all, or if you overtighten the bolts to the flange holding it, you can break things. Think of those bolts more as alignment rods than anchors. The toilet must sit flat without rocking and the bolts should NOT be used to pull it down and seat it on the wax...you push the toilet down with your weight, then just snug up the bolts. Then, in most places it's required, you caulk around the front 3/4's of the toilet to prevent crud from accumulating underneath it and to help hold it in place. On something like a tiled floor, the caulk holds it much more than the bolts - porcelain to porcelain has very little friction and it doesn't take much to slide the toilet unless it is caulked in place. Use something like PolySeamSeal rather than silicon, so you can remove it without major hassles down the road.