Yes. You'll need to file an amended income tax return. You'll go back to the "Stocks, Mutual Funds, Bonds, Other" interview and correct that particular trade to reflect the correct basis. A typical "same day" sale results in a small loss due to selling commissions and fees.
This FAQ
http://turbotax.intuit.com/support/iq/Amend-a-Return/How-to-Amend--Change-or-Correct--a-Return-You-A...
will get you started on the amending process.
Tom Young
