I wonder what's causing this kind of problems and how to get rid of it. I tried to remove doubles and recalculate normals but these don't work. Any ideas?
Realtime renderer alpha artifacts
I think is related to the way polygons are drawn from the viewport engine.
To make the viewport render faster, the algorithm doesn't take into account whether a face is in front of another, leading to such artifacts. It happens quite the same in other realtime engine like Sketchfab viewer (see Why order matters section). This is pefectly understandable: the software should perform much more calculations if it should not takes into account only the face visibility but also the alpha depth.
See in the image sequence below how till I'm stacking the quads one above the other in order (latest on top) all works well, while switching their position leds to wrong results:
At the end, it seems more important the order in wich faces are "stored" in the file than the alpha&camera-distance factor. The artifacts in the picture comes from a that. Some faces are drawn in front of other even if they are behind.
There are some freshly implemented viewport alpha options for cycles, like BGE's ones, but despite their promising names, they don't work (yet).
Here's a reel of the options in BGE:
While in Cycles you can only see a difference when "Add" is enabled
I personally think developers are "working on it".