Currently you can enter an invoice adjustment without selecting a fund. In this case the adjustment amount will be shown in that specific invoice but will not be respected in the funds table on the acquisitions start page. To replicate: Add an adjustment to an invoice. Do not select a fund when adding the adjustment. Look at the acquisitions start page - the adjustment amount is not included in the 'Spent' column.
Actually I mean that it should be mandatory to select a fund. So you shouldn't be allowed to save an adjustment without first selecting the fund.