The feature cannot be added because it's geometry collapsed due to intersection avoidance.
see also http://i.imgur.com/xiiI7NE.png
What happended: Let's say I have 2 layers. To the first I enabled the snapping tool with avoid Intersections. So far so good.
Now I added a polygon to the layer 2. That layer has nothing enabled in the Snapping tool. But as you can see from the error above, it's broken. So I'm not able to create a polygon on top of the layer 1. Is this intended? I mean where does the avoid intersection option make sense if this is intended?
This is more a general question, because with disabling the avoid intersection I was able to create a polygon on layer 2.