I can't say that I've ever seen it happen, partly because I don't press my seams open, and if I did I would not stitch over them. It seems very logical that if the needle hit directly on one of the threads holding the seam together, that thread would be broken. Except for that possibility, the cutting of the threads might not be an immediate thing, but the result of wear over time, assuming that it does occur. What is clear to me is that if you stitch directly over a seam that was pressed open, you are not stitching through any of the fabric in the top, so that means that the stitches are not holding the top together as well as if the stitches went through the fabric.