Is it safe to modify and return a bag that was given as input?

[Jonathan Coveney]

I feel like the answer is that it is not safe, but I'd like to make sure.
IE is the following ok, and if it is not, why not?

public DataBag exec(Tuple input) throws IOException {
  DataBag bag = (DataBag)input.get(0);
  long index=0;
  for (Tuple tuple : bag) {
    tuple.append(index++);
  }
  return bag;
}

Appreciate the guidance.