If you hang out with Dwayne long enough, he will eventually tell you that he doesn't know how to read or write, however he is perfectly capable of sending text messages with proper spelling and word use.
In "She's A Keeper", while Gordon is moving Grace Ancelotti from the safe house to the trunk of the car, he yells out that Grace bit him. How could Grace bite Gordon when her mouth is gagged, her hands are tied behind her back (so she can't grab him to bite him), his hands are on her waist and he's behind her the whole time?
On Public Liberty Radio's 'The Pacemaker' show, Waylon Mason explains that his mother died during childbirth, but later refers to her as "leaving him in dirty diapers all day", despite the fact she died during his childbirth, which is verified when he and his father home-cremated her.
On the Richard Bastion Show, he says we don't have many great traditions left, but when a caller says no one cares about holidays and what people are doing wrong, Richard says "what are you talking about? We have a lot of great traditions left."
In the cutscene for the mission 'Museum Piece', Niko enters through a swinging door. However the behaviour of this door changes throughout the cutscene. When Niko attempts to leave, Ray is able to slam it closed, and at the end of the cutscene the door is able to stay in the one position to allow us a view of Ray after Niko leaves.
When traveling down Babbage Drive in Central Alderney, it is a 4-lane road, like a highway, and there is the north and south bound lanes. Like a highway, the north and south bound lanes have two lanes each. When looking at either highway lane, it has the double solid yellow line, which would mean that the northbound and southbound lanes are in fact, a two-way road, and that Babbage drive is in fact, two different roads.
On the radio you can hear Lazlow say "you get in high-speed
accidents, you go "dude, I lost my right arm" and your friends would say "dude,
who cares? you can drum."" This is clearly a reference to Rick Allen, but Rick Allen lost his left arm, not the right.
In the mission "Flatline", a character is on a life support machine, as is stated when you attempt to turn it off. However, the character is well able to speak, something which would be highly unlikely if his condition necessitated the machine.
In the mission 'Hating the Haters' Bernie says he has been getting attacked at the park and needs the help of a good fighter like Niko, but if you get into a fight with someone before you reach the park during the mission, Bernie will also start to fight the pedestrian and he fights just as well as Niko. While fighting, he will also speak with a different style of voice.
In the cutscene for Gerry McReary's mission 'I'll Have Her', a sign in the background says "Liberty City Correctional Facility Duty Officer Rotation"; despite Gerry being in the Alderney State Correctional Facility.