I have this problem on all my Mac's with the Calculator Widget but it is fine once I click it, it seriously annoys me, does this happen to anyone else?
Yes, I've had it happen too. Strange, but nothing to be really worried about, it seems to fix itself.
I might be wrong, but I reckon it'd be a cache error in which case trashing everything within the DashboardClient folder from the Caches folder and restarting the Dock/Dashboard might fix it.