On page 6 of the game's manual, there's a news article about how a protest group called A.R.S.E. (American Road Safety for Everyone) is pushing to get motorcycles banned from Liberty City, and might also be responsible for getting bicycles banned from Liberty City a few years prior. This explains the absence of bicycles and motorcycles in Grand Theft Auto III.

The Hero Garb was originally red and blue. It was based on a Mexican wrestling outfit.

The RC Goblin, a little toy copter from Vice City, was to return but was cut. It can still be spawned with trainers, but cannot be controlled.

A functional Tear Gas weapon exists in the game, but it normally goes unused. It can be acquired with trainers.

Early beta screenshots portray Salvatore Leone with a slightly different appearance from that seen in the released game.

A pre-release screenshot showed the Avenging Angels wearing red berets instead of black caps.

Data for The Hunter helicopter is present in the game's files, and can be spawned through the use of trainers. Its weapons are working.

The title of the mission Crazy '69' refers to the film Kill Bill, where the main character, played by Uma Thurman, fights a group of Yakuza gangsters known as The Crazy 88. She uses a katana, just like Toni.

After killing Paulie Sindacco in the mission "Dead Reckoning", an elegant tombstone bearing his name appears in Liberty City Cemetery, engraved with the message "VIVA LAS VENTURAS BABY". This could be a reference to the 1964 film, "Viva Las Vegas" or to the 1963 song, "Viva Las Vegas".

The Bickle'76, a modified Taxi unlocked via the Taxi Driver side-mission, is likely named after Travis Bickle, the protagonist of the 1976 film "Taxi Driver".

Toni Cipriani was likely made in homage to Tony Soprano, the main character of the American television drama The Sopranos. Aside from their names, both characters also share ongoing problems with their mothers, who also ordered hits on them.

The PSG-1 reuses a beta model of the weapon found in the files of the PS2 version of Grand Theft Auto: Vice City.

There is unused text which states "ENGLISH.gxt/TURPASS/~W~Massacre Passed!"
"Massacre" was most likely an early name for the Rampages, although it's possible they were something else.

An early preview screenshot showed Miguel wearing an outfit identical to what he wore in Grand Theft Auto III.

The name of the mission, The Guns of Leone, is likely a play on the film ''The Guns of Navarone''.

The 'Dragon' jumpsuit is clearly modeled after the iconic yellow-and-black jumpsuit worn by Hai Tien/Billy Lo (played in part by Bruce Lee) in The Game of Death.

The Underwear and Overalls do not show Toni's chest hair, despite it being visible on Goodfella and 'The King' outfit.

In the PSP version of the game, if the player continues the side mission, Car Salesman, after level 6 and reaches level 9, a snowy weather effect starts to appear and continues appearing in further levels. This is possibly there to make driving more difficult. The snow disappears if the player fails or cancels the mission, and Car Salesman cannot be played again afterwards, so you can only see this once. This weather effect does not appear in the PS2 version of the game.

In the parking lot behind the Liberty Tree building in Bedford Point, there is a hidden sign, showing a handwritten message and Rockstar Leeds' signature smiley. The PlayStation Portable version says: "Hello again!", while the PlayStation 2 version says "You just can't get enough of this alley, can you?". The sign can be found by jumping over the walls around the lot using a tall vehicle.

In most of the mission 'The Guns of Leone' Salvatore is equipped with a Micro SMG, however in the cutscene at the end he is equipped with a Shotgun.