Weird black dots on clock in panel

[gibbs1984]

Weird one this, occasionally a couple of black dots will appear on the clock in the panel, as per the screenshots below which I had to take with my phone because the screenshot app doesn't pic it up strangely. Now when I hover the mouse over the clock the black dots then disappear.

I originally thought this was because I had a programme that wasn't full screen and was sitting behind the clock and shining through but that isn't the case.

Obviously this isn't a big problem because it doesn't change/effect anything but thought I'd mention it as a bug:

Linky.