True, at least for formal proofs. I was thinking more of traditional mathematical proofs, which are often as hard to formalize as programs are to prove correct. In both cases, convincing yourself that the proof or program is correct involves reading it and seeing whether each of its parts follows from or fits into the other parts to make a coherent whole.

Of course, there are ways to fool the reader, but I think people learn to avoid writing in ways that are likely to mask errors (e.g., not reusing the same symbol to mean completely different things, both in proofs and in programs).