I was playing a match in an 8 ball league last season (just a regular night, not the tourney or anything) in which I ran down to the 8, and had basically a hanger straight into the side for the win. I was cueing off the rail, and someone walked behind me and bumped my cue. My ferrule nudged the cue ball and it rolled all of an inch. I touched nothing else, and the cue ball touched nothing at all.
My opponent said, with a very a-hole tone about him, "Man, that sucks. It's a foul though." He took ball in hand, and with a wide open table, ran his remaining four balls, and the 8, for the win.
Did I foul? Yep, technically I did. I just put myself in his shoes, and every single time, I see myself looking the other way on the foul and allowing my opponent to shoot. Everyone on his team told me they would have let me replace the cue ball and shoot the shot as it was before I was interfered with. I just happened to run into a hard-nosed jerk that day.