Formal verification would solve everything. It's just that whoever is using the software actually needs to understand the specification, but when there is some trusted base of I/O primitives (like "read a file"), such things become trivial to check; even Haskell has such things in a limited fashion via SafeHaskell and to an even lesser extent via its IO monad.

The specification for a text editor would be much simpler than an implementation. For example, efficiently searching for a substring is non-trivial, but a specification is easy. So, all that I would be interested in, is a proof that "eval(optimized_substring_search needle haystack) = eval(easy_substring needle haystack)", for example. Obviously, there are many thousands of such theorems that would have to be done to clone Emacs, but at least a new release wouldn't contain bugs anymore (wrongly specifying something would happen, but it's much easier to write a specification of desired behavior than to find the exact bug in some mess from someone else, because it conflates implementation and specification in the first place).