SymptomsAfter using a key combination (for example Cmd-Shift-Control-4 to take a screenshot) hitting any keyboard button when in virtual machine activates a shortcut involving a Control/Shift/Command/Option key and the button you have pressed. For example clicking S opens "Save as..." window (which normally opens with Control+S).
WorkaroundThis is a known, so called "Sticky key" issue. Parallels Development is aware of it. For now there are 2 workarounds possible:
1) Click the faulting key (Control in our example) and "S" will behave as it should.
2) Go to Parallels Desktop preferences and disable key mapping for key which behaves abnormally ("Cmd+S > Control+S" in our case).