Mythical Sheng Long
In the 1991 Capcom arcade fighting game "Street Fighter II," at the end of each match, static images of the two competitors would appear on screen, the loser all bloodied and the winner spouting boasting dialog via a text caption. In the U.S. version, one of Ryu's dialog cards said, "You must defeat Sheng Long to stand a chance." Some players took that, along with the absence of any Sheng Long in the game, to mean that Sheng Long was a hidden character you could unlock.
Electronic Gaming Monthly even published a story that confirmed the cheat, saying that Sheng Long would be unlocked if a gamer played as Ryu, got to the end of the game undefeated with no damage, and, in the final battle with M. Bison, neither dealt out nor received any damage before each round timed out. After all that, Sheng Long would reportedly throw M. Bison off screen and fight you in his place.
Predictably, the story was another April Fools' gag. Electronic Gaming Monthly even mocked up pictures of the imaginary character on screen with M. Bison and Ryu. The tip was presented as a write-in from W.A. Stokins from Fuldigen, HA (or, sounded out, "waste tokens from fooled again, ha").
In actuality, Sheng Long was one of Ryu's punches (shoryuken) badly translated from Japanese into a Chinese dialect and then again into English. In the original Japanese version, the dialog read something like, "If you cannot overcome the Rising Dragon Punch, you cannot win!" For the Super Nintendo port of the game, probably to avoid similar confusion, the dialog was changed to read, "You must defeat my dragon punch to stand a chance!" Electronic Gaming Monthly even reprised the trick for an April 1997 article detailing the Sheng Long character in the upcoming "Street Fighter III."
As happens sometimes with video game myths, Sheng Long was eventually incorporated into the lore of the game when Nintendo included him in the instruction manual for the Super Nintendo version, which stated that he was Ryu's former martial arts master.