Closing an application sometimes leaves a gap and/or overlying task fields on the taskbar. I would say having lots tasks and multiple rows increases the chance to trigger this, but I also get this with only two or three tasks and a single row. The fact that I'm using a slow onboard graphics card with desktop effects enabled could also be relevant. If I had to guess, I would say it looks like a race condition. This bug is present since at least KDE 4.4. Reproducible: Sometimes
*** This bug has been marked as a duplicate of bug 224447 ***