[Agentlien]

Yes, that's why you need to make such a room when making a map with a mirror in Duke Nukem 3D.

Modern games also do a lot of tricks when it comes to mirrors. Real reflections are expensive. Though they're becoming increasingly common thanks to the ray tracing hardware on recent graphics cards.

As a player I recently noticed that in The Last of Us Part 2, when you're in front of a mirror the windows give off an unnaturally bright light to hide that the outdoors area wasn't reflected.