Originally Posted by FBinNY

The downtube and lower head lug are in tension. When they fail, the front end drops down and pushes the front wheel forward. What happens exactly depends on the strength and ductility of the top tube, and strength of the top/head joint.

From first hand experience, I've seen one where the top tube held and bent upward in a way that the rider could control the bike and avert a crash. Note that front wheel braking in this instance would improve the conditions.

But IMO that was a unique event, and I've seen dozens of frames where a lower joint failure led to an immediate and catastrophic upper failure, with high potential for face plant. In the cases I've seen, the extent of injury to the rider varied, and I have no direct first hand knowledge of anything causing grevious injury or death, but it remains a real possibility.

While I'd ride a bike with some lower head tube damage, such as a crash related upset at the downtube, I wouldn't ride one where I felt the risk of imminent failure was high, ie. visible cracks propagated more than 10-15% (if that). If the blue line in pure photo repreresnts the extent of an actual crack, I'd consider the frame toast (or a candidate for a retubing job)