It is possible to assign a command to a modifier key by dragging and dropping the command from the command list to the keyboard layout. If you then attempt to use the command via the shortcut, it won't work.

You can't assign a command to a modifier key when double clicking it, so you shouldn't be able to do it via drag/drop either.

1. Install Unity
2. Launch Unity
3. Create a new project with any settings
4. Go to Edit > Shortcuts
5. Select any command and drag/drop it onto a modifier key in the keyboard layout e.g. shift, control, alt

Expected result:
The user should not be able to assign commands to the modifier keys.

Actual result:
The shortcut binding is successfully created, but the command will not work when the shortcut is pressed.

Tested on Windows.

Occurs on:
2019.1.0a5, 2019.1.0a12