Was it for the Additional Medicare tax on high earners (for wages)?
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Questions-and-Answers-for-the-Additio...
If you have a lot of investments or high income there is an Additional Medicare tax on Schedule 2 line 8 which goes to 1040 line 15. That is offset with an extra amount on line 17 with withholding.
