I'd estimate about half of all programming is throwing up barriers to writing buggy code.

There's a constant tension between building fast and right (or should be if you're not fucking up).

Sure. Regardless of how wacky your definition of "functional" gets, it is possible and relatively easy to write bugs in Python, Scheme, Haskell, or OCaml; all of these languages confuse `x + y` and `x - y`. Idris, Agda, or Coq can catch that mistake, but still suffer "Boolean blindness" and other traditional problems.

There are plenty of bug classes which are trivial in any language; plan interference is a good example. Languages provably cannot avoid these bugs entirely, just make them less easy.