Pressing the 'Super' (a.k.a. Windows) key while in rectangle-select mode will lock the user in the 'select region' mode. Pressing escape or enter will not have an effect
STEPS TO REPRODUCE
1. Press Print key
2. Select 'Rectangular region' -> Take a new screenshot
3. Press the Super key
The program is locked in the region-selection mode. It remains responsive to mouse actions, however keyboard inputs are ignored. Since neither escape not enter terminate the selection process, the system is effectively locked. The behavior is reproducable on my system. Fiddling with Ctrl+Alt+[F-key] seems to crash X when returning to the X session the bug was produced in. I also observed an uncalled system reboot.
Anything from 'nothing' to 'the start menu opens'. Personally, I tried to take a screenshot of my start menu.
Linux/KDE Plasma: Antergos/arch Kernel 4.19.2-arch1-1-ARCH
KDE Plasma Version: 5.14.3
KDE Frameworks Version: 5.52.0
Qt Version: 5.11.2
You can draw a box and double-click in it to escape when this happens.
*** This bug has been marked as a duplicate of bug 374009 ***