Mask on forehead=panic. Where did this mask signal originate?

[KenE]

It is a good example of a great reward for little effort.
The classic curve of likelihood v impact looks like this:
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A common example of low likelihood but high impact is Nuclear War...top left on the chart. The other extreme is high likelihood but low impact -- exchange of nasty letters between ambassadors -- lower right on the chart. the point is tht most thing fall on this curve somewhere. The point is to pay attention to things that are high impact, especially if you are diving. Also, pay attention to things that are moderate impact but way off the curve to the right, i.e., anomalously high likelihood even if only moderate impact.

A scuba example of high impact but low likelihood is complete loss of gas (e.g., gas bloockage, hose burst); we mitigate this by using doubles and an extra second stage, for example. the key is that we look at the difficulty of a mitigation measure: if it is easy to mitigate a high impact problem, then do it.

Mask loss is somewhere in the medium to high impact range....depends on if you can surface or not, for example. So we mitigate with a low difficulty measure: carry a spare mask.

"the point is tht most thing fall on this curve somewhere. "

This is not true (IMHO), the curve is the cutoff, everything to the right of the curve are items that should be mitigated. Some things fall to the right and some to the left. Impact is also relative to the type of diving. I would argue that a mask loss on a typical recreational dive has a low impact and a very low likelihood and therefore falls to the left of the curve where no mitigation is required. The curve is open on both ends because something with a very high likelihood but very little impact or a very low likelihood and high impact does not require mitigation. Each individual adjusts the curve based on their risk management tolerance.