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NES - Did You Know Gaming? Feat. Caddicarus
 
There is a bearded man in the front row of the crowd that flinches/ducks the moment the player needs to punch their opponent to win by KO against Piston Honda and Bald Bull.
Contributed by Kakariko Kid
The HUD and boxing rings in the Japanese version of Punch-Out!! have different color palettes than the ones found in the English version.
Contributed by Ophl
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In the Japanese version, Piston Honda and King Hippo's skin tones are pinkish. In the English release, their skin tones were changed slightly to be a slightly darker yellow tone instead.
Contributed by Ophl
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There are two opponents that were removed from the NES port; Pizza Pasta and Kid Quick. It's unknown why they were removed.
Contributed by GoldenFloor
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In the Arcade version, characters that resemble Mario and Donkey Kong can be seen among the audience if the camera pans over to the left side of the ring. One that looks like Donkey Kong Jr. is also present further in the background, sitting beside another Mario look-alike.
Contributed by Blase005
It was Nintendo of America president Minoru Arakawa who decided to pursue a deal with Mike Tyson to use his name and likeness for the US release of Punch-Out!!. He believed that Tyson's name would help sell the game to customers still wary of the video game market after its crash only a few years earlier.

Although Tyson had not yet won a championship, Arakawa had attended one of his matches and was impressed with his power and skill. This gamble would pay off well, as Tyson would go on to win his first title fight against Trevor Berbick for the World Boxing Council (WBC) heavyweight championship, making him the youngest heavyweight champion in history. Mike Tyson's Punch-Out!!! would sell more than a million copies before the licensing deal expired.
Contributed by Boyobmas
Great Tiger is the only fighter who's introduction appears at the start of Round 2.
Contributed by game4brains
Professional boxer Mike Tyson, who the US version of the game is named after, and appears as a fighter in, had never played the game until 2013, 26 years after it was first released, which he did on Fox Sports.
Contributed by KnowledgeBase
Many of the original Punch-Out!! and Super Punch-Out!! cabinets were converted in to "Play Choose 10" cabinets, as they had the same design. Nintendo even sold conversion kits to help arcade operators. Because of this, finding an original intact cabinet of either game is very rare.
Contributed by gamemaster1991
Makoto Wada has said that Mario's appearance as the game's referee was never meant to be. He placed the character in the place of the referee without approval from higher up in the company, and when it came time for final testing, no one said anything about removing him and he was left in.
Contributed by gamemaster1991
Entering the code 800 422 2602 at the main menu will give you a busy signal. This code was Nintendo's Customer Service phone number.
Contributed by Pogue-Mahone
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The game was originally a Japanese only Gold Cart game that was won in a "Golf U.S." Tournament, where players would send in their high scores to Nintendo in order to enter the contest. The main difference between this and later copies of the game was that the final boss was Super Macho Man, and not Mike Tyson/Mr. Dream.
Contributed by gamemaster1991
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There is a hidden circuit in Punch-Out!! called the "Another World Circuit", which can be accessed by inputting the code 135 762 4680 on the password entry screen, then pressing A, B and Select simultaneously. The fights (in order) are King Hippo, Great Tiger, Piston Honda (Second Fight), Soda Popinski, Bald Bull (Second Fight), Don Flamenco (Second Fight), Mr. Sandman, Super Macho Man, and finally, a title bout with Mike Tyson/Mr. Dream, strangely labelled as "ANOTHER MCHAMPIONN." There is no extra content in this mode, but it does serve as a way to get to the later fights more quickly.
Contributed by DoubleYooToo
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The Russian fighter Soda Popinski was originally named Vodka Drunkenski. They changed his name to avoid controversy. Most of his dialogue still makes it out as though he is in fact supposed to be drunk.
Contributed by EliasAnsel