When a pass of CreateTransaction fails to select enough bitcoin to meet the nFeeNeeded the next pass through may select a far smaller collection of inputs.  Currently it will continue to use the old fee amount based on the larger set of inputs, which can cause a non-trivial over-payment.

This may be the cause of the behavior reported here: https://www.reddit.com/r/Bitcoin/comments/5ltw5n/bitcoin_core_v0131_sends_enormously_high_fee/

Long ago fixing this was tricky due to a catch-22 between signing the transaction and knowing how much fee was required.  But after #9465 the whole algorithm will run using a dummy signature, so when there is already a change output fixing up the fee after finding a sufficient transaction should be a matter of a ~2 line change.

if (change_position->nValue >= MIN_FINAL_CHANGE + additionalFeeNeeded) {
                        change_position->nValue -= additionalFeeNeeded;
                        nFeeRet += additionalFeeNeeded;
                        break; 
                    }
                }
                
                nFeeRet = nFeeNeeded;
                continue;
            }
        }
        if (sign)
        {
            CTransaction txNewConst(txNew);
            int nIn = 0;
            for (const auto& coin : setCoins)
            {
                const CScript& scriptPubKey = coin.first->tx->vout[coin.second].scriptPubKey;
                SignatureData sigdata;
                if (!ProduceSignature(TransactionSignatureCreator(this, &txNewConst, nIn, coin.first->tx->vout[coin.second].nValue, SIGHASH_ALL), scriptPubKey, sigdata))
                {
                    strFailReason = _("Signing transaction failed");
                    return false;
                } else {
                    UpdateTransaction(txNew, nIn, sigdata);
                }
                nIn++;
            }
        }