I have had my JT slotted mat for about 8 years and have never had a problem I have used my 45mm Olfa cutter and the smaller one also. I made sure I start in the teardrop shaped slot before I start cutting. I hold my cutter straight. If you slant a bit to the left or the right, you will not get a true measured cut. I had a student who slanted her cutter to the left. She could not understand why her blocks were coming out the wrong size, even though she had measured her seam allowance which was exactly the 1/4 in. She says now when she starts cutting, she hears me say, "cutter straight up".