If a law is passed that makes changes that are retroactive to your 2017 tax return (such as the percentage of AGI used for determining the amount of medical-expenses that are deductible) and you filed your tax return without taking that change into account, the IRS might detect the error and refund a larger amount. If they don't, you can always file an amended tax return to make the correction up to 3 years after the due date of your tax return
