Say a steel frame has a developed a crack in the lower head tube that extends about halfway around the lug / head tube from the rear. Obviously this lug / joint will fracture at some point, causing a break across the lower part of the head tube. I’m curious: does anyone have any knowledge or experience with what happens when a head tube fractures like this while a bike is being ridden? My guess is that it would be fairly obvious when it breaks completely through, but that the bike would remain rideable at least long enough to pull over without a serious crash.

Here’s an example image: