The IRS has two categories of dependents: Qualifying Child and Qualifying Relative. If you are 21 y/o and not a full-time student, they cannot claim you as a qualifying child. This leaves only the qualifying relative option.
- You must be unmarried (or if married, not file a joint return).
- You must be a U.S. citizen.
- You are not the qualifying child of anyone else.
- You must either live with them as a member of the household all year -or- be related to the taxpayer.
- You must have gross income less than $3,900.
- They must provide more than half your support during the year.
If you made more than $3,900 last year, they are automatically disqualified from claiming you as a dependent. That's the simplest thing to prove, and the IRS will be able to verify it quite easily. You will not be in trouble for filing and claiming yourself, as you are legally allowed to do so. They will not be in any legal trouble either, but will be required to re-pay any portion of their refund they received by claiming you.
I wouldn't go so far as to say the software allowed them to do it, they could have entered incorrect information or filed using another method. The bottom line is that they filed before you.
My recommendation is to claim yourself, then print and mail your tax return. If you made more than $3,900, the IRS will allow you to claim yourself. Your refund may be delayed a bit, but that's all. They will then contact your parents and let them know they need to repay a portion of their refund. It will really be quite simple, but will take a bit longer than if you had been able to e-file.
I hope this is helpful. Please let us know if you have further questions.