When you set the capture mode to "Rectangular Region" and then go to take a snapshot, as you move the mouse the outline of the region to be selected lags behind where the mouse pointer actually is. Unfortunately, the behaviour doesn't really show up when I try recording it to video using qt-recordMyDesktop, so I can't show it to you. Reproducible: Always I'm using an ATI Radeon graphics card with the non-proprietary driver, which I've heard has some performance issues, so that might be the cause of the problem.
Hello! Sorry to be the bearer of bad news, but this project has been unmaintained for many years and I will be closing this bug. Spectacle is the replacement for ksnapshot now. Please test again and file a new bug for Spectacle if you still have issues. Thank you!