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.