I tried to upvote a comment on SO, but when I moved the mouse over the greyed out up arrow, it disappeared. I can click on the blank space and an upvote is counted. Now using the links at the bottom of the page I open up Server Fault, go to a comment on a question there and the comment flags work as expected. Meta, SuperUser also OK.
System: WinXP Pro
Java: Sun JRE 1.6
Browser ID: Opera/9.80 (Windows NT 5.1; U; en) Presto/2.2.15 Version/10.00
Is this a bug?