Grand Theft Auto: San Andreas
On a back wall of the underground impound lot in Los Santos, there appears to be a police officer repeatedly punching and beating a pedestrian. The pedestrian will have his/her hands raised, as if he/she is surrendering. Another police officer can be seen standing nearby, holding a pistol in case anything gets out of hand.
It is most likely an easter egg made by Rockstar, to humorously illustrate the alleged police brutality that occurred in California during the 1990s.
What is strange is that, even if you kill both policemen, the pedestrian will not move from their spot, and will just stand there with their hands up until they are eventually killed. However, injuring the pedestrian with a weapon will make them put their hands down and either fight back, or run in place. It is still unknown why the pedestrian does not make any attempt to run away or defend itself, but it could be simply because the pedestrian was not programmed to do so.
An unrelated case of possible police brutality can be witnessed when the player has a three star wanted level or higher. The gunner inside the perusing Police Maverick will shout "I'm going to kill this guy!" but another officer, presumably the driver, says to make sure he kills "the right one." This may imply that the Police Mavericks indiscriminately fire and harm civilians.
Grand Theft Auto V
Similarly, in GTA V, the LSPD have been accused of brutally raping women in discrete locations. The acclaims seem to be partially admitted on their Bleeter page, where they claim to hire prostitutes for oral sex. The brutal rape can be seen in an image on their Bleeter page, however no other images of the aforementioned brutality can be found or seen.
This myth may be an Easter Egg, as it could be referencing the large amounts of police brutality in 1992, most notably the Rodney King case, in which four police officers were acquitted after they were videotaped beating Rodney King in Los Angeles in 1992. The acquittal of the officers was a primary factor for the 1992 Los Angeles riots.