There is a part of the panel, the "task manager", where opened applications are shown. In this part, there is a button for every opened application.

By clicking a button, usually the related application gets focus on the desktop. But this does not always happen: there is a bug preventing this basic behaviour in some circumstances.

How to reproduce:
1. Have a slow system (artificially reduce CPU clocks, make the system busy, etc...)
2. When you move your mouse over a button in the panel, a small "preview" of the application is shown: it takes up to half a sec to render
3. Click on the button BEFORE the preview has been shown
4. The click gets ignored

In other words, if clicks come too fast to the applications buttons, some of them just end up being ignored. I am not talking about a click-spam but about real-world scenarios.

I would expect plasma to queue those clicks or even better to give them priority over different tasks (like preview rendering).