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.

{
    LOCK(cs_wallet);
    
    if (setKeyPool.empty())
        return GetTime();
    
    CKeyPool keypool;
    CWalletDB walletdb(strWalletFile);
    int64_t nIndex = *(setKeyPool.begin());
    if (!walletdb.ReadPool(nIndex, keypool))
        throw runtime_error(std::string(__func__) + ": read oldest key in keypool failed");
    assert(keypool.vchPubKey.IsValid());
    return keypool.nTime;
}
std::map<CTxDestination, CAmount> CWallet::GetAddressBalances()