suddenly unworkable square bug...?

[Wretched Gnu]

I'm playing Civ3 on a Mac; in my current game, one of my cities suddenly can't work on one of its most valuable squares: the square has that brown line around the perimeter that tells you the square is being worked by another nearby city -- but it isn't. As a "workable" square, it *is* shared with another city I have nearby, but neither one of them is working the square and neither one of them can get to it.

There are no other cities around, and both cities are well within my national borders, so I know there aren't any "hidden" enemy cities that could be claiming the square... any thoughts?

I must say that, otherwise, the game runs beautifully on my wife's ibook...

thanks,
--wn