Great thread here but have I missed this one - flipping a seam the opposite way to get it to nest and causing it to twist in the middle?
When I moved from a third floor apartment to the first floor, I just picked up my mat with project and open rotary cutter to carry down the stairs. Started on my way down, tripped, and the cutter fell all the way down. Thank goodness no one was below me and the quilt police were busy elsewhere. :-)