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 (!CCryptoKeyStore::AddWatchOnly(dest))
        return false;
    nTimeFirstKey = 1; 
    NotifyWatchonlyChanged(true);
    if (!fFileBacked)
        return true;
    return CWalletDB(strWalletFile).WriteWatchOnly(dest);
}
bool CWallet::RemoveWatchOnly(const CScript &dest)
{
    AssertLockHeld(cs_wallet);
    if (!CCryptoKeyStore::RemoveWatchOnly(dest))
        return false;
    if (!HaveWatchOnly())
        NotifyWatchonlyChanged(false);
    if (fFileBacked)
        if (!CWalletDB(strWalletFile).EraseWatchOnly(dest))
            return false;
    return true;
}
bool CWallet::LoadWatchOnly(const CScript &dest)