Unthinkable does not have anything to do with probability. Rather, it’s something the author would prefer to not think about.

Also, you saying it’s common is a judgement call and not a true frequency call.

It can be both unthinkable and uncommon and possible.

I think the right phrase is something like "worth thinking about" rather than "common". It's like the "in the very unlikely event of a water landing..." you hear on airplanes. It is very unlikely but there's something relatively easy you can do that will improve your chances if it does happen.

Either way, it's disturbing that this might be "worth thinking about".