Leaks

There have been several leaks of ARMS content before it was officially announced.

Fighter leaks
The following fighters appeared briefly in the 4.12.17 Nintendo Direct, but Nintendo had not yet released any details about them. (Kid Cobra) (Byte) (Twintelle)

Later, they were leaked again when a video demonstrating the three of them battling in a Team Fight was accidentally released before the ARMS Nintendo Direct.

Hedlok leaks
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 has since been taken down and replaced. Later, Nintendo Life referred to the final boss as Max Brass in an article.