If the only change on your tax return is the loss of your $4000 personal exemption because you were claimed as a dependent by someone else, then an additional amount of up to $4000 of income becomes taxable. If you are in the 10% tax bracket, that would be up to $400. There is a possibility that the IRS might charge penalties and interest. They would notify you after you print and mail the amended return and pay the ordinary income tax due if you need to pay anything additional.
There is a possibility that your parents claimed you by mistake. Here are the tests from their perspective that you had to meet for them to be able to claim you as a qualifying child dependent:
1. The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of the year, (b) under age 24 at the end of the year and a full-time student, or (c) any age and permanently and totally disabled. A student meets the full time student requirement if they were full time for at least 1 day of 5 different months during the year.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you (or your spouse, if filing jointly) unless disabled.
If you didn't meet the qualifying child tests, then they might have been able to claim you as a qualifying relative dependent if you met the following tests:
1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.
2. The person either (a) must be related to you or (b) must live with you all year as a member of your household.
3. The person's gross taxable income for the year must be less than $4,000 in 2015.
4. You must provide more than half of the person's total support for the year.
5. The person must be a U.S. citizen, U.S. resident alien, U.S. national, or a resident of Canada or Mexico.
If your parents claimed you by mistake, then they would have to amend their tax return and remove you as a dependent. You would not have to make any changes to your tax return.
