Ubuntu's main menu panel applet has some drawing issues with various panel sizes, particularly when the panel is positioned at the bottom of the screen.
The applet has the Ubuntu logo as its icon as well as an arrow (from the current theme) indicating that it expands when clicked.
The following problem occurs both with the Human theme and the Clearlooks theme and no other theme-related errors of this sort occur on my machine so I think it is safe to assume the problem is with the applet.
The error I am reporting here has to do with that arrow: Its position seems very strange when its panel is positioned at the bottom of the screen and it does not sit well with other panel applets. With small panel sizes it overlaps the Ubuntu logo on its right side and with big panel sizes the arrow is displayed so far away from the icon that it is invisible. Evidently, the arrow's size / position is not taken into account when the entire main menu applet's size (and thus the position of the next applet over) is determined. At those larger sizes, the main menu's arrow actually ends up being drawn behind a neighbouring applet such that it is either cut off by that applet or it appears attached to that applet. Either way ends both in confusion and a burning pain in the eyes.
A guess for what is going on: The arrow is drawing in a correct place when the panel is aligned to the top or left, but the applet does not take into account the panel's position when drawing so the arrow is drawn a distance /away/ from the icon instead of a distance onto the icon (as it is with the panel at different alignments).
Sorry about my weird description. I know very little about how panel applets work!
I will post screen shots below, which should show what I'm on about a lot more clearly. Keep an eye on the arrow attached to the main menu icon.
My suggested fix is to remove the arrow since it is very small (hard to notice) on the default panel layout, though another possibility is to have an arrow always intentionally drawn beside / above its icon (as with the Deskbar applet). Also, if my guess above is accurate, then I suppose the fix is actually fairly simple.
Oops, I missed one!