OS: windows 7 64bit, CPU: i5-3550 GPU: ATI HD7850
I found that it seems to be a particular range of angle between faces in wich the rays are able to miss the triangles.
Fireflies or worse are the consequences.
Exact steps for others to reproduce the error
1.put the camera inside a cylinder (32 vertices)
2.rotate the cam so it is looking at the round part of the cylinder
- set the background shader strength to 5000
4.render it on 200 samples
It should show some fireflies all over the image.
This problem occured to me on an extreme case as I built a sealing for a room.
I'll attach a simplified file of it:
btw. the "a" version of blender 2.75 has some problem with my GPU so I renderd with CPU.