Its easier to shoot people in the chest than in the head.
If you blow up a few helicopters a tank will blow up your car. (On that note I think the GTA wanted system was based off ot the original Blues Brothers movie.)
If you try to board a pirate ship the crew will try to kill you, but if you hide in a crate they will take you to the bottom of the ship where you can wait for the crew to go to sleep.
Never shoot something with a rocket launcher if its nearby, the screen turns red and you dont know if you died until the red goes away.
The knife is always a 1 hit kill but you have to shoot them twice in the head with a gun to kill them, except for sniper rifles which always miss the head and hit the air to the side of their sholder (but count as a chest shot) and you have to hope they don't hide before you finnish them off.
If you stab someone in the foot the kife is still a 1 hit kill.
You cant kill people on the other side of a sheet metal fence but if they back up to it you can kill them by shooting the part sticking through the fence.
The higher you get the smaller area you can hit with a sniper rifle due to the fog of war.
You can hide from people in ghille suits but if they see you theres no way you can hide from them even if you were to run into the thickest wooded area you have ever seen iin your life and hide under a bush... it just helps them get a headshot when they come around that tree.
You can always tell who the bad guys are due to their theme music or because they wear alot of black, usually with red or blonde hair.
If someone has weird colored eyes they are a super zombie who is about to try to kill you, most likely will succeed unless you have already played through the game once on easy mode and beat it within 5 hours and unlocked the infinite ammo weapon that you never have to reload.
Never trust a blonde cop in sunglasses.
If you kill a guard in 1 hit all the other guards still aggro when they see you, even if you were completely undetected by said guard and left no evidence. They always know its you and wont stop chasing you until you run halfway across the map, however when you go back into town they won't realize that you killed the guard until you talk to one of the guards in the town or tresspass in the guard house.
You can steal priceless objects and get away with it if you jump down a long hole, the guards will never know you did it.
If you play a musical instument all bees and rodents in the area will die and turn into watermelons.
Famous gun stores dont ask for an ID and dont make you do any paperwork.
If the police are chasing you, you can spray paint your car and they will go away if you drive carefully even though you were the only person in the garage.
When you kill someone the body part you shot them in flies off of their body, but if you shoot them in the chest all of their limbs disappear. However sometimes when you kill someone their body blows up and you have to loot the little pices, but the items on the body will be unharmed.
If you subtley put an active grenade in theit pocket they immediately run around screaming and blow up no matter how stealthy you were in putting it in their pocket.
If you find a green ball and eat it you can only see in black and white but nothing can kill you.
If you humor little girls who sound like old men they always keep their word.
If you stike a deal with a bunch of vampires they teach you how to drink blood.
But most importantly, if you die you don't respawn until the next round.