If an applet's size changes enough, it can cause the panel to stick to the wrong side of the screen.

Steps to reproduce:
1) Create a panel on the top of the screen.
2) Disable the 'expand' option for the panel, and move it up against the right edge of the screen. (IOW, a horizontal corner panel, right-aligned)
3) Add the System Monitor applet
4) Open the System Monitor applet's preferences dialog, and increase the applet's width until it is too big for the screen's width to accomodate
5) Decrease the applet's size back to a reasonable value

Expected results:
Panel is still aligned against the right edge of the screen

Observed results:
Panel is aligned against the left edge of the screen.

100% reproducable.

I know this is a rather obscure process to duplicate, but it can happen easilly in regular usage. The notification area can expand to include more icons, then shrink when icons are removed. Applets that include status text can change length very easilly.

More annoyingly, the sensors applet behaves rather poorly, and produces a large text message if it's unable to connect to the hddtemp daemon. If the daemon is inactive, this can occur several times a minute.

In this case, the gnome panel frequently and persistantly jumps from one side of the screen to the other every time I fix it :)

I haven't yet been able to test under 2.13.x.