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.

/**
     * populate vCoins with vector of available COutputs.
     */
    void AvailableCoins(std::vector<COutput>& vCoins, bool fOnlyConfirmed=true, const CCoinControl *coinControl = NULL, bool fIncludeZeroValue=false) const;
    /**
     * Shuffle and select coins until nTargetValue is reached while avoiding
     * small change; This method is stochastic for some inputs and upon
     * completion the coin set and corresponding actual target value is
     * assembled
     */
    bool SelectCoinsMinConf(const CAmount& nTargetValue, int nConfMine, int nConfTheirs, uint64_t nMaxAncestors, std::vector<COutput> vCoins, std::set<std::pair<const CWalletTx*,unsigned int> >& setCoinsRet, CAmount& nValueRet) const;
    bool IsSpent(const uint256& hash, unsigned int n) const;
    bool IsLockedCoin(uint256 hash, unsigned int n) const;
    void LockCoin(const COutPoint& output);
    void UnlockCoin(const COutPoint& output);
    void UnlockAllCoins();
    void ListLockedCoins(std::vector<COutPoint>& vOutpts);
    /**
     * keystore implementation
     * Generate a new key
     */
    CPubKey GenerateNewKey();
    void DeriveNewChildKey(CKeyMetadata& metadata, CKey& secret);
    
    bool AddKeyPubKey(const CKey& key, const CPubKey &pubkey);
    
    bool LoadKey(const CKey& key, const CPubKey &pubkey) { return CCryptoKeyStore::AddKeyPubKey(key, pubkey); }