If a user sends assets to a lightbox that were not intended for that collection, they now have the option to undo the action. To send assets to a lightbox, you can either drag and drop, or select Send from the options ☰ menu.
Once the assets are sent to the new or existing lightbox, a popup notification will appear at the top of the screen. There you will see the option to Undo:
Selecting Undo will remove those assets from the lightbox.