Region capture mode leaves artifacts
Binary package hint: ksnapshot
Press "Prt Sc" button (or just launch ksnapshot), choose "Region" mode for the "Capture mode". Then click and drag the selection over any region of your screen; doing this on a white background will demonstrate the problem best. Notice the grey (might be a different colour dependent on your X11 cursor theme) artifacts that follow the path of your cursor as you expand the region. When you save the image, these artifacts are saved along with it.
I'll attach an example image of the artifact in question.
Using ksnapshot 0.7 in Feisty (latest).