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.

{
    nIndex = -1;
    keypool.vchPubKey = CPubKey();
    {
        LOCK(cs_wallet);
        if (!IsLocked())
            TopUpKeyPool();
        
        if(setKeyPool.empty())
            return;
        CWalletDB walletdb(strWalletFile);
        nIndex = *(setKeyPool.begin());
        setKeyPool.erase(setKeyPool.begin());
        if (!walletdb.ReadPool(nIndex, keypool))
            throw runtime_error(std::string(__func__) + ": read failed");
        if (!HaveKey(keypool.vchPubKey.GetID()))
            throw runtime_error(std::string(__func__) + ": unknown key in key pool");
        assert(keypool.vchPubKey.IsValid());
        LogPrintf("keypool reserve %d\n", nIndex);
    }
}
void CWallet::KeepKey(int64_t nIndex)