This is a bug with the scenario exactly same as in the following bug:
6594793 Nimbus L&F: JTabbedPane Tabs have a thick black line around them.
The above issue was only partially fixed in this pit and the black line still appears for cases where the tab has a component set over the tab.
In the attached test case the tabs located at the tab index 2,4 and 5 have a component placed over them with the following code:
tabpane.setIconAt(2, new ImageIcon("image1.png"));
        tabpane.setTabComponentAt(4, p);
        tabpane.setTabComponentAt(5, comboBox);
If you comment out these 3 lines the bug dissappears: If you uncomment any one these 3 lines the bug is reproduced.
This is seen with the curent pit build 6u10 b07 on all Oses, with nimbus L&F.