Hey, It's That Voice!: Bear Hugger is voiced by Richard Newman (who coincidentally lives in Bear Hugger's home province of British Columbia), who is well known for playing Rhinox, another big guy. Not only that, he voiced a certain M.Bison who said "yes".
Piston Honda is voiced by Kenji Takahashi, who also voiced Abel from Street Fighter IV.
Spanish players may also recognize the voice of Don Flamenco being Juan Amador Pulido, a Spanish voice dubber for such roles as Tom Tucker and Cleveland of Family Guy, Shadow the Hedgehog from Sonic X, Gabriel of Dexter and The Captain of WALL•E.
Charles Martinet provided pretty much all the voices in the SNES Super Punch-Out. He is more recognizable as the voice of Mario and Luigi (among others).
In the NES version, a camera held by an audience member at the top-right corner of the screen will flash when Bald Bull's bull charge can be countered.
Little Mac (or someone that looked like him) was going to be the player character in the SNES Super Punch-Out!! before being replaced by the blond boxer for unknown reasons. Nintendo has flip-flopped over whether this character is Mac with a makeover or an entirely new boxer for several years.
No Export for You: Inverted. Japan didn't get a release of Super Punch Out for SNES, except for a very limited edition in March 1998.
Talking to Himself: Great Tiger in Title Defense mode makes several illusions of himself between rounds. They hang out on the ropes talking like they're on break.
In a more straight example, Doc Louis and Mr. Sandman are both voiced by the same guy.
These two characters are also the only ones that refer to Little Mac as "Mac baby".
In an interesting coincidence, the initial barrage of punches Mike Tyson hammers you for in the NES version lasts 91 seconds, the amount of time it took Tyson to knock out contender Michael Spinks in 1988.
The only boxers to appear in all three console versions of the game are Bald Bull, Super Macho Man, and Mr. Sandman. Due to Arcade Super Punch-Out being a Mission Pack Sequel, no boxer has appeared in all games.