Weird Render transparency artifact As you can see in the picture below, I have just rendered a frame of an animation and a triangular piece is missing. You can download the .blend and try it for yourself - it happens on some frames(like 24 in the example) and on others it doesn't, from what I've seen about every 4th frame.

Interesting to note is the fact that the fog, which I have strengthened to see the effect, still renders correctly, so it's only the plane with the image





EDIT: It went away after moving the image about 25 more units away from the camera - no idea why, and I would still highly appreciate suggestions.
This is in fact a very old known issue with Blender Internal renderer (caused by numerical imprecision), see e.g. this nine years old report.

Workaround of triangulating the buggy mesh (using e.g. the relevant modifier) seems to be working quite nicely, when that issue shows up.