So here is the thing: It was presumably relatively easy for you to come up with that scenario, which you called "not-unlikely". Then what you do is you put that scenario into your risk analysis when you're designing the authentication architecture, and figure out mitigations to make sure that particular mistake becomes (very) unlikely.

The notion that "it could easily happen" that is being brought up throughout this thread should really only suggests that people aren't doing even rudimentary security assessments (or, hopefully, they're not working with security sensitive software).

If you can't solve it technically, you solve it through processes and training. Same goes for any other industry -- if a construction worker said that it's just one bad morning away from dropping a two tonne girder on a playground, we would never accept that. Or a pilot crashing an airliner into the waiting hall when they're supposed to land. Somehow it seems that large parts of the software industry simply hasn't reached the level of maturity we expect from pretty much all other industries.

Facebook is an enormous company. They should be able to have entire departments working on these topics. It's not a one-person hobby project we're talking about.