i keep a mirror tile on my quilting table, everytime i walk away, change bobbins, shut the machine off, turn it back on after 2-3 inches i use that mirror to look at my back and make sure nothing is amiss...
after screwing up one time and spending 6 hours taking a 12" strip of quilting out of a quilt i learned my lesson, i stop and look pretty much everytime i start and get going again. so sorry this happened to you, it is definitly difficult to remove a mess from the back of a quilt :(