I looked and looked but I couldn't find it anymore. Somebody did an experiment on the "flipping eggs" theory. I thought it was a thread on bp.net but I can't find it in the search.
If I remember correctly, there was no conclusive evidence that flipping an egg harms the baby.
But, just by logical thinking, I think if the egg rolls over when the snake is already big enough in the egg could cause the umbilical cord to get wrapped around the snake.
My observation with maternal incubation is that the eggs are "stuck" together so that it is not an easy task to get them flipped.