When one hovers over a tab name on user profile / activity page, the name gets underlined twice, in addition to changing color to red. I suspect this may be a CSS bug, because having three effects on hover (color change, highlight on the line below, and underline directly under the word) seems too many.
Seen in Chrome 47 and Firefox 43. For a live demonstration, see the profile of this valuable contributor.
(kos pointed out this report, which is related but focuses on the bounty indicator. There has been no response to that one.)