Cycles runs multiple times when sampled motion blur is checked
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If you have the Sampled Motion Blur checked on the Blender Render with a number of samples specified, then switch to Cycles to render, Cycles will re-run the entire rendering the number of times specified in the Sampled Motion Blur box.

For example, if I have the sampled motion blur checked in the Blender Render and specify 16 motion samples, then on Cycles I have specified 30 samples in the integrator settings, Cycles will render the scene for the 30 render samples, then start over, rendering the 30 render samples again. It will re-run the entire render 16 times, the number of motion samples specified in the Blender Render panel. If the Sampled Motion Blur is un-checked in the Blender Render panel, then the Cycles render is run, it only runs through the 30 render samples and stops as expected.

If the rendering tried to perform motion blurring, it would be understandable for the rendering to run multiple times. Unfortunately, it just re-runs the rendering of the scene multiple times without micro-stepping the movement to simulate motion. If you didn't realize you had that setting checked, you may expect to run a 1000 sample Cycles render, only to return to your computer to find that the 1000 samples finished and it re-started, completely wasting the time and processing it took to reach those 1000 samples.

No blender file is needed to illustrate this - it works like this every time on my 64-bit Blender 2.62 Windows installation. I'm running Win7 64-bit Professional, 4GB RAM, ATI FireGL 512MB, Core-2 duo processor.