If I have OS X display the time in the menu bar with blinking separators, the seem to blink inconsistently. I've taken a 32 second video of the effect. It's happening on two of my MacBook Airs, 13" Mid 2011 model. Running MacOSX 10.8.4. One is used at work and the other is my personal machine.
I know that this is a rather small issue but I'm still very curious on what might be causing this problem.
Looks like it's fixed in Mavericks!