Have a strange occurance when RAM Previewing an animation with motion Blur applied. I have used this effect many times...but for some reaosn this project and my last started having this issue...therefore think it's something preference/setting wise.
When I run the RAM preview, the animation doens't blur as it should but gives a weird duplicted type effect...which is definantely not what I am after. I have attached an image of it a couple of frames into the animation.
might also be an opengl issue, in preferences>previews uncheck the opengl option for previews, however, if your motion is generated by a random() expression that kind of double image can occur with motion blur.