> No, I am saying that is that it is easy to not verify something because you don't know the requirements up front.

Granted, but "this spec doesn't attempt to cover [X]" is very different from "this spec claims to cover [X] but actually doesn't, in ways the authors are unaware of." The former is quite common in formal specs with machine-verified correctness proofs and I've never heard anyone call that a "spec bug". The latter is not common in them at all. Perhaps you didn't intend this yourself, but the way people generally talk about "there could be a bug in the spec" is usually used to imply that the latter case happens often enough to make formal verification comparably effective to other defense mechanisms, when empirically formal verification is far, far more effective than any other bug prevention mechanism--to the point that people struggle to find even individual examples of these kinds of spec bugs.

> There are invariants which you can assume that are always true. If they aren't true for whatever reason you can abort and later track down what caused you to enter this state. It could be as simple as some logic bug or obscure as hardware malfunctioning and causing a bit flip (at scale you need to deal with hardware misbehaving).

Yes, and stating these invariants precisely is generally exactly what you need to do as part of writing the formal spec; getting them right is equally hard in both cases, so it seems weird to me to say that it's easier to check the invariants at runtime than to get the spec right. In fact, many formal specs have invariants that are exactly of the form "assert that if I ran this line of code, it would not throw an exception." Moreover, you can use much more "expensive" invariants when you're creating a spec, since you don't actually have to check them at runtime, which in practice lets you catch far more and more subtle bad states than you can with software checks. For example, your proof state can include a full history of every action taken by the application in any thread, and prove that invariants on them are maintained during every instruction, even when those instructions proceed concurrently and might witness partial or stale views of the latest values in memory; storing and checking all of this information on every step on an actual machine would be wholly impractical, even for high level languages and non-performance-critical software.

Obviously, things are defined according to some base model of the world, so if you are concerned about hardware bit flips (which in rare contexts you are) your spec should take that into account, but the vast majority of bugs in modern applications are not bugs of this form, and most defensive runtime checks are equally susceptible to hardware issues or other unexpected environment changes.

> Bugs are found in the process of testing too.

Yes, and I never said testing was bad. Formal verification and testing are complementary techniques, and tests are important in a formal verification context because you should be able to prove the tests will pass if you've gotten your spec right. However, them being complementary techniques doesn't mean that they are equally effective at reducing bugs.