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There have been several leaks of ARMS content before it was officially announced.
Nintendo Direct leak
The following fighters appeared briefly in the 4.12.17 Nintendo Direct, appearing in the background behind the Set ARMS menu. At the time Nintendo had not yet released any details about them, instead focusing on Min Min during the Direct. Helix was later revealed in a video on Nintendo's YouTube Channel, and the rest were revealed during the ARMS Nintendo Direct on 2017-05-17.
Team Fight leak
Later, the three fighters from the May Direct were leaked again when a video demonstrating the three of them battling in a Team Fight was accidentally released before the ARMS Nintendo Direct by the YouTube Channel SwitchForce, where they were officially revealed and detailed.
Gameplay footage of a battle against Hedlok was accidentally leaked before Nintendo had lifted the review embargo on ARMS.
Max Brass leak
A screenshot of a battle against Max Brass, the final boss of Grand Prix, was posted by the review site Stevivor before ARMS was released. It was taken down and replaced to avoid breaking their embargo. Later, Nintendo Life referred to the final boss as Max Brass in an article, to avoid further revealing information.