While holding Ctrl, most keys won't insert text into a text input (like QLineEdit or QTextEdit). However, the following list of keys is found to ignore whether Ctrl is pressed on X11:

!#$%&*(),.<>+?'":;

even though the following list is ignored while Ctrl is pressed, just like all alphanumeric characters:

@^_{}[]/|~`

Comparing to other toolkits, the behaviour of Java, Mozilla XUL, GTK and Windows (tested Notepad) seems to be to consistently ignore text input while Ctrl is held. Not ignoring the text is problematic when for example a shortcut is assigned to Ctrl+. (or any of these keys), cause then when the shortcut is disabled for some reason, the key isn't eaten and will suddenly insert the text instead.

The issue exists at least for Qt X11 as well as Qt on Windows, though current behaviour isn't 100% the same.