What Games have scared you but you just dont know why?

[Alby_da_Wolf]

Alby_da_Wolf said:

 

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Edit: if I must limit myself to "I don't know why", then, some strange atmospheres in Ecstatica II, Hexen I, Heretic II and Thief I,II,III

And also the atmosphere in the lost pueblo near San Andreas fault in Duke Nukem 3D was quite scary.