I'm thinking fluke. A lot of these are running on minibikes and carts with no suspension and this is the first time I've seen this. Could something have hit the carb before or during the ride that may have cracked it? (then vibration caused it to break completely) The more I look at how it broke, the more I wonder if the flange faces (carb and manifold) were flat; if either of them had a high spot (likely at the bottom considering how it broke), it would have stressed the carb casting when the nuts were tightened. Another thing I notice is how low the exposed portion of the o-ring is hanging, makes me wonder how it could have been in the groove; if it wasn't, and was pinched between the flanges instead, that could have caused it (same as a high spot), although I'd think that would have caused a vacuum leak and a resultant running problem. Just thinking...