I noticed a "flashing" effect in the path bar in Nautilus when there are many subdirectories open and when the window is of a certain width.

Actually, I am only assuming this is Nautilus. I found the bug on the default file manager of the Ubuntu 13.04 Unity desktop, which they call "Files". Subsequent searching led me to believe that this file manager is Nautilus; apologies if this is filed in the wrong place. My version of "Files" is 3.6.3. My system has the latest software updates, but 3.6.3 seems a few versions behind the latest Nautilus version of 3.10. I'm not sure if this is because of different numbering between GNOME and Ubuntu, or something else.

To get the flashing effect, I created new subdirectories until the directory bar almost filled up the alloted space in the window. That is, until just before the first directory disappears and is replaced with a scroll arrow. Then I resized the window, making the width a tiny bit smaller until the first directory is just about to disappear. Then I moved the mouse over the path bar, and the path bar starts to "flash". This "flashing" appears to be the path bar rapidly moving left and right, not being able to settle on a directory. The window width has to be very precise for this bug to appear; the area seems to be 2-3 pixels wide.

This is probably easiest explained with the attached video (a 6.1 MB OGV) . Let me know if you have any problems viewing it.