I had two blow outs on Conti 4000s last year, both whilst descending a hill, one whilst on Season of the Mists", brand new tyre totally shredded on that one (but that damage was caused by rolling down the hill as I tried to stop with the tyre off the rim) and the second on a one day 170 mile coast to coast, descending a very steep hill with too many blind ninety degree corners to ease off on the brakes and free run, into Pately Bridge. I put it down to the rims overheating though which effect the molecular structure of the tyre itself making it easier to come out of the rim and not the tyre itself.