In the mission "Friend Request" Mike leaves the back pack behind in the office after rigging the prototype to explode. This would have been really stupid considering the police would collect it and analyze it for evidence.
Even though it was established that Lester can only walk with the aid of a cane, during the mission "Cleaning Out the Bureau" when he and Michael return to the factory, Lester is able to walk up stairs perfectly without the use of his cane.
When getting the gold from the Union Depository the bank manager mixes Empirical and Metric units. (ounces and Kilograms). Because this games takes place in what would be the United States it is more likely the 4 tons mentioned would be short tons. 1 short ton is equal to 2000 pounds or 907 Kilograms.
In the mission "The Wrap Up", Michael De Santa, Steve Haines, Dave Norton & Andreas Sanchez are engaged in a Mexican Standoff with Merryweather and the FIB. Steve's gun used in the standoff was an automatic pistol. However, when Steve shoots Sanchez in the head, the gun is set to a single shot.
Along the highway in rural areas, traffic cops (Called the San Andreas Highway Patrol in-game) can be found standing and watching the highway with police motorbikes next to them. However if driven past with a wanted level or provoked, they'll usually and strangely Comendear a civilian car rather then use there bike (unless they're sitting on it when you go past).
The items Nigel has Trevor collect for him include a gold tooth, underclothes, a dog collar and a golf club. All of which would not fit in the small paper bag Trevor gives to Nigel upon their next meeting.
Runway number in the game are not correct. Airport runways are numbered according to what heading an aircraft using the runway will be on. For example runway 18 will be used by airplanes headed south, heading 180. The other end of the runway will be numbered 36.
Guards at the prison in Blaine county and soldiers at the U.S Air force base can be seen using AK-47's. The air-force would NEVER issue these weapons to soldiers and it's very unlikely that a US prison would issue these weapons to guards.
The North Yankton State Patrol (NYSP) are seen using sedans and SUVs that are based off vehicles from the 1980s despite them appearing in the section of the game that takes place in
2004. This is incorrect as police departments across the U.S. in 2004 were using the more modern post-1998 Ford Crown Victoria (and many of them still are).
The character of Impotent Rage is credited twice, with two actors named as voicing the character. This is because 'Frank Simms (II)' voices the cartoon character, and Graham Rowat voices an actor dressed as Impotent Rage outside the Hardcore Comic Store in West Vinewood.
The song "I Get Lifted" by George McCrae incorrectly includes a writer credit for K. Murray, & E. Sermon. New York rapper Keith Murray sampled the George McCrae original as it plays within the game for his own track called "Get Lifted".
The GTA games are set in a universe where places have alternative names to their real world counterparts (New York City is Liberty City, Los Santos is Los Angeles, Hollywood is Vinewood etc). Most of the songs on the in-game radio stations are real life songs not written specifically for the game. Some of these songs, such as Hollywood Nights by Bob Seger and Los Angeles by X, reference real life places in their lyrics. If these songs existed in the GTA universe, they would be called Vinewood Nights and Los Santos respectively.
The goof items below may give away important plot points.
After Trevor kidnaps Mrs.Madrazo both Micheal and Trevor go into hiding and will be attacked if they attempt entry to the city, yet Franklin is free to move about the city, even contacting Micheal and Trevor while they are in hiding. He is never bothered by Martin Madrazo or his men. As a known associate of Micheal Franklin would be the next logical step in getting to Mike or Trevor yet Martin never even bothers to try to call Franklin.