My panel is laid out from left to right as follows: K Menu, a few launchers, task manager, pager, system tray, clock.

When a new icon appears in the system tray, the following happens very quickly:
1) The system tray area briefly expands to the right, becoming superimposed slightly on top of the clock.
2) The panel widgets to the left of the system tray resize to make room for the expanded system tray area.
3) The system tray moves to the left, taking the new elbow room given to it by the resizing widgets.

However, after the tray icons all move to the left, the rightmost part of the system tray (usually the popup arrow) is still visible superimposed on top of the clock (see attached screenshot). This graphical glitch does go away when the clock display is updated (usually once every minute when the time changes). Clicking on the clock also chases off the superimposed arrow.

Reproducible: Always

Steps to Reproduce:
1. Log into KDE.
2. Start an application that causes a new tray icon to appear. For me this would be Thunderbird (brings up message indicator) or anything that causes notifications, such as copying a file.
Actual Results:  
Arrow-impaled clock.

Expected Results:  
Healthy, non-impaled clock.

This behavior has been happening for a while, for sure since the KDE 4.10 series and continues to happen with KDE 4.11.1. I've just been too lazy to report it until now ;)