When selecting and deleting an item in the Timeline or Workbench, following rules apply:

Deleting an Item in the past: Anything (Playlists, Videos, Graphics, Events) that lies completely in the past cannot be deleted.
Deleting an Item in the present (= On Air): Playlists and Graphics that are currently On Air may be deleted. A warning will be shown.

Videos that are On Air cannot be deleted but skipped by clicking the Play Next button.
Deleting an Item in the future: Anything (Playlists, Videos, Graphics, Events) that lies in the future may be deleted without warning.

If no warning is shown when deleting present items you can enable all warning dialogs again in the just:play Preferences. Click the Reset button as displayed below.

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