According to several sites - none of which offer sources or citations - the Arc De Triomphe de l'Étoile was damaged on or near the same day that the Battle of Verdun began in February 1916. These sites, including this one, claim that the Sword held by the Republic on the La Marseillais relief broke off.
Some accounts go on to say that the damage was immediately concealed behind a tarp to prevent people from treating the damage as an omen portending disaster:
The sword carried by the Republic in the Marseillaise relief broke off on the day, it is said, that the Battle of Verdun began in 1916. The relief was immediately hidden by tarpaulins to conceal the accident and avoid any undesired ominous interpretations.
Note: The footnote citation in the Wikipedia article links to the page I linked above, which offers no sources or citations
Is there any evidence that this story is true, and if so, when and how did the damage occur?