Something wrong with oversampling

I’m trying to render a scene (two layers, no compositing): even if oversampling is on, I get results like the attachment images (first with 8, second with 16 samples). I just can’t figure it out, why the oversampling doesn’t apply in this particular scene.

EDIT: Now I found out what causes the problem: The room walls, roof and floor (single object). When I removed it, the antialiasing works. But why is that so? The object has a bevel modifier. I don’t understand.

Attachments



Found the guilty: the material had Ztransparent button on. Stupid me.