You have selected multiple objects and when trying to use the Shift key to deselect objects, it fails to deselect any objects, and you have Raster Design installed or are using Map 3D or Civil 3D.
This problem can be caused by one of two things (it a toggle that’s turned on)
First things to try:
Type in MAPIOPTIONS (Civil 3D or Map 3D only)
Navigate to the General Tab and uncheck the Shift+Left Image Select
If behavior continues, type in IOPTIONS
Navigate to the User Preferences Tab and uncheck the Shift+Left Image Select
That should restore the preferred behavior to allow you to use the Shift key to deselect objects.
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