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.
