--- Operating System, Graphics card ---

Mac OS X 10.8.2, AMD Radeon 6759M


--- Blender version with error, and version that worked ---

Error: r54336 (official test for 2.66), No error: r53177 (official release 2.65a)

The error appears in the testbuild. I've rendered an animation of 500 frames using compositor scale in the official release version, and there is no problem.
Blur and/or colormix appears to have a different effect between the versions also, but it's hard to describe in terms of a bug.

--- Short description of error ---

Using scale node in compositor, sometimes frames are only half-rendered. The right half part of the frame does not get rendered when this happens.


--- Steps for others to reproduce the error (preferably based on attached .blend file) ---

1. Enter compositing screen.
2. Check use nodes.
3. Delete Renderlayer input.
4. Add input->image.
5. Set image (I used a 3840x2160 png).
6. Add distort->scale.
7. Connect image to scale.
8. Set scale: x=0.5, y=0.5
9. Connect scale to composite (output).
10. Set renderscale to 100% (1920x1080).
11. Hit F12 to render a few times. (After less than 10 attempts the error is visible for me.) Alternatively try to render animation, and watch the frames being rendered.