Yes, Challenger - although program management knew they were violating a launch constraint (temperature), and it was the low temperature that produced the conditions necessary for SRB failure.

As with any aerospace mishap, it's a chain of events, not just one cause.

Yes, challenger. The O-ring failed, creating a gas exhaust that almost instantly destroyed the main propellant tank.

Yes, 50% of shuttle losses were due to SRB failures (Challenger)

That's exactly how Challenger was lost.