For many years, Saul's sole proprietorship and his related Form 1040 have had a number of AMT tax preferences and AMT adjustments. He has made the AMT calculation each year, but the calculated amount always has been $0. Saul's regular taxable income and the AMT adjustments and preferences for 2017 are the same as for last year. Yet, he must pay AMT this year? Explain how this could happen.