I had a customer complain about this once as well, and I told him it was part of the process, and sometimes it happens due to the pressure needed to clear the ink out of the screen. What I started doing to prevent this is to cut a piece of "chip board" 2 1/2" wide x 20" and tape it to the underneath side of the screen. It absorbs most of the initial down force of the squeegee to shirt. every time I use that, I get no marks.
But Mr. R, in this case is being a bit over dramatic on this. Just my .02