If I'm going with childhood:

In Duke Nukem 3D you could give money to a stripper and she would flash her tits at you. Mind you this game came out in 1996.

For a child that's like 9 or 11 seeing

A: strippers,

B handing money to said strippers and then

C, strippers pressing their tits at you on screen.

Is simply something I would never expect to happen in a videogame. When my friend showed me I didn't even comprehend it what was happening? What did I gave her? What is she doing? I was too young to realize the implications.

Objectively I find the physics engine in Half Life 2 has to be the most impressive thing ever. Nothing for me comes really near it because it's such a big thing that a lot of games ultimately copied or find a thing that's normal nowadays.

Stacking single bricks to increase weight on one end of a seesaw, boxes turning over and falling down if stacked improperly, breaking individual planks into smaller pieces, pushing barrels down hills, the whole gravity gun, throwing balls into hoops. Sharp and heavy objects do more damage than small and soft objects.