What does it mean when the United States flag is upside down?

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Tags

1. #displayed
2. #american
3. #should
4. #instances
5. #distress
6. #pondered
7. #union
8. #according
9. #danger
10. #property
11. #meaning
12. #symbol
13. #extreme
14. #except
15. #others
16. #never
17. #states
18. #signal
19. #united
20. #upside