For the last number of major releases plasma shell "randomly" stops updating the state of its plasmoids in the panel (the clock stops, load viewer (if enabled) stops, the task list doesn't update). The system is still responsive to user input, so clicking on an application in the task section of the panel does open (or bring forward) a window, but not the correct window. The alt-tab task switcher works correctly and lists all windows.

Manually killing/restarting plasmashell fixes the problem temporarily, but it comes up again soon. The issue is much more likely to occur on high system load, even when system monitor is not included. It looks like some sort of race condition.