When tabs spill over into a second row because there isn't enough horizontal room, the second row is hidden partially behind the background for the tab above it. See cutoff tab to left and right of "Part Numbers" below. (note - part numbers can be seen because it is the selected tab)
Menucool has rewritten the McTabs to Responsive Tabs. The new release is v2014.12.23 that is supporting the responsive layout. So using the latest version of the Responsive Tabs will resolve your issue.
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