By far, the MOST COMMON MISCONCEPTION is that you holed the tube with your irons. The fact is, MOST of the pinched tube events come from when the tube is down in the drop-center portion of the rim, 180 degrees from where you are working. When you pull the bead over the rim, the other side is forced down into the drop and cuts the tube. The goal is to have enough air in the tube to keep it UP OUT od the drop-center channel.
Too much is just barely enough.....