I don't cut anything on the mat until I removed all other fabric off the mat. Learned that the hard way when I was "just going to trim that one last seam" and ended up cutting a corner off a top that "snuck" under my piece. Of course it was something I had NO MORE fabric to fix. Other than that, I will rip a seam (and even recut pieces) twice. If at the third attempt, the piece still won't be perfect, it stays the way it obviously wants to be. Like someone said: Done is better than perfect in that case.