When I use the vector blur node sometimes, and there the objects in the scene aren’t moving very much, the blur is really really distorted. Even though there should be no blur. The max and min velocity are both set to 0.
I had that problem with the explode modifier as long as not every particle was emitted and I couldn’t find a way to get rid of that… seems buggy.
I had that same feature happen to me http://www.youtube.com/watch?v=aUyZn9PEL6g