The function fund doesn’t check if the amount to fund is multiple of rewardPerBlock. Thus, if the amount to fund isn’t the multiple of rewardPerBlock, then the remainder of amount/rewardPerBlock gets lost in the farming contract and can’t be used as a reward. The ideal behavior is to refund the excess fund to the funder since those funds won’t be able to be used by anyone and will get lost.

It would be better to disallow funding of amount that is not perfectly divisible by rewardPerBlock .