HA!!, I did see a Mossberg rib come loose. I was keeping score, many, many years ago, and when I looked over at the shooter with the Mossberg getting ready to shoot, I saw the rib had slid forward about 12". The shooter never noticed it.
I had a 1998 vintage MX10 topsingle where the rib broke. The piece that broke off was the part just below the notch for the front sliding adjuster, similar to the description given by ray119 above.
I don't know why Perazzi chose to leave a sharp corner in the cutout as most engineers know a corner is an area increased stress concentration. Yes my gun was ported, but I do not think that was a contributing factor. My gun also had an extra adjustment notch added by one of the barrelsmiths, making the metal a little thinner where it broke.
I have noticed that on newer guns, the cutout is milled from the side and the notch now has a radius, much stronger.