This happened to me on one ride last year. It was the middle of fall and I figured the end of the riding season so I was in no particulare rush to replace my tyres which had about 5000 miles on them (sometimes I can be so cheap). Luckily it happened in the rear. I had flatted earlier that day from some glass along the side of the road and replaced the tube with my spare. I was then on a long downhill doing about 40MPH when my rear tyre blew. I was going straight and unweighted the rear a bit to keep my rim from damage. After inspecting the tyre, I realised that what had happpened was that the earlier puncture had spread across the thin bald tyre casing and exposed the innertube to the road which promptly exploded and ripped the carcass right down the middle. I vow never to let my tyres go that far again.