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 (age != 0)
                        age += 1;
                    dPriority += (double)nCredit * age;
                }
                const CAmount nChange = nValueIn - nValueToSelect;
                if (nChange > 0)
                {
                    
                    
                    
                    CScript scriptChange;
                    
                    if (coinControl && !boost::get<CNoDestination>(&coinControl->destChange))
                        scriptChange = GetScriptForDestination(coinControl->destChange);
                    
                    else
                    {
                        
                        
                        
                        
                        
                        
                        
                        CPubKey vchPubKey;
                        bool ret;
                        ret = reservekey.GetReservedKey(vchPubKey);
                        if (!ret)
                        {
                            strFailReason = _("Keypool ran out, please call keypoolrefill first");