Fortunately, no. The amount of the benefits includible in gross income may be zero, somewhere between zero and 50%, or somewhere between 50% and 85%; 85% is the maximum taxable percentage. The exact amount depends on the amount of your net benefits, your tax-free interest from municipal bonds, and your overall income.
On Form 1040X, you may file an amended return within a three-year period to claim a refund or correct a mistake made on an original or previously amended return.