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It should not be possible to delete a Segment or any other asset (Marketing email, Marketing page, Marketing form) which is used in Live Customer Journeys. If you do so, the Journey is set to an Error state and can't be edited or deleted anymore. Only MS support is then able to help and to reset the status.
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Completed
Administrator on 3/30/2020 4:48:41 PM
We have introduce a warning mechanism when stopping live records: Before stopping email messages, forms, or segments used by live customer journeys, the user sees a warning message to help prevent the user from inadvertently corrupting an entity used in a live journey.