If you put your mouse over the System Load plugin and let the tooltip pop-up on the CPU usage, and the CPU usage goes from 9% to 10% or 10% to 9%, the right black border to the tooltip get some breaks in it. This also happens when going from 99% to 100% and 100% to 99%.
Additional information: This seems to be not only a Panel tooltip problem, but a system-wide tooltip bug. Whenever a tooltip that is currently being displayed changes size, it gets messed up a little. (Of course, not many programs change thier tooltip size, thus the reason this probably hasn't been brought up before.) I have only confirmed this bug with the System Load plugin (with the CPU usage graph), since it seems to be the only current program that changes the text in it's tooltip in such a way. I can attach screenshots (they will be tricky to do) if needed, but anyone should be able to reproduce this. Jsut keep your mouse over the CPU usage graph of the System Load plugin and you will see.
As you may have guessed this is not an xfce problem, but a problem with gtk. We cannot solve this.