No? That's the whole point of formal verification?
You can even kind of retrofit this to C. The classic example is "sel4". You just need a set of proofs that the code doesn't trigger UB. This ends up being much larger and more complicated than the C itself.
You can fail to verify something which you actually wanted to verify (i.e you made a proof of something else instead of the thing that mattered). See WPA2 KRACK as an example.
You can even kind of retrofit this to C. The classic example is "sel4". You just need a set of proofs that the code doesn't trigger UB. This ends up being much larger and more complicated than the C itself.