If the quoteToken is rebase token, the priorBalances may change due to rebasing or airdrop. It may result to an incorrect accounting. However, whether it is exploitable depends on the Bond markets logic.
With the current logic, it just checks whether the balance is increased more than the inputAmount_, so it is harder to exploit, compare to the alternative logic of using the difference in balances as the input amount. However, it also introduces the possibility of paying the users less than they deserve.
