In GTA III's Definitive Edition, Rockstar decorated the inaccessible Upstate mountains with trees, decorative houses, and even fake road lights — but the entire area remains non-solid and unreachable by any means.
Source: Upstate Liberty — CC BY-SA
GTA III has invisible towns that exist only to crash your game.
GTA: Liberty City Stories
Rockstar cut an entire swamp island owned by Avery Carrington from Vice City
GTA: Liberty City Stories
Is GTA IV's infamous Ratman actually a resurrected GTA III ghost?
GTA: Liberty City Stories
A car that never existed in GTA III drove fans crazy for years — and Rockstar trolled them hard.
GTA: Liberty City Stories