Oops - that's a session verification error:
An Error Has Occurred!
Session verification failed. Please try logging out and back in again, and then try again.
And thanks for pointing it out. Back on the first day that I installed this theme, Midnite discovered the same error was occurring with the "Mark as read" button on the BoardIndex page. I fixed that one (the code for the link wasn't verifying a member's session) and I thought I'd caught all the other instances where a similar button shows up, but apparently I missed one.
(I even confessed to Midnite that I was afraid I wouldn't remember where all those buttons show up - mostly because I don't use them.) But I'll get on that right away...
Could it be the doing of that old Grinch, Diabolos?
We've theorized more than once that the forum is haunted, but it never occurred that he could be causing the unexplained weirdness. Hmmm...