...I've had one alloy bar failure at the stem junction. it did not completely separate, but
I still pretty much lost control of the bike, but did not crash, so I guess you get "some" warning.

Nevertheless I found it very disconcerting, and generally try to use bars with a steel sleeve
at the point where they run through the stem. I see guys using carbon bars around here, and
I think that they are way more adventurous than me.