We actually it is quite hard to describe my problem with only using words. 1
My problem is this weird wavy contour. Without the plane underneath the can, the reflections look fine (like in the bottom part), but even when I use the standard Glossy BSDF on the can and a plane with Diffuse BSDF this problem occurs. Using a higher roughness on the can doesn't change much except that the border between the black part and the correctly reflecting part becomes less prominent. I am using two area lamps that are lighting every other object correctly.
In this I am using a HDR image for world lighting and a plane without any specific material and now the bottom part doesn't reflect like it should.
Also, the mesh isn't faulty or something in that particular place and if I look at the can from another angle the intersection moves.
I would like to know what I did wrong and I'd be glad for your help.