There seems to be a bug in low quality rendering. The top part of the image shows a screen shot; the lower part shows the render of it in quality 1 (mine go from Q0 to Q3). As you can see, two objects are completely missing: only their shadows survive.
In Q0 the same happens, but without the shadows.
Design and render made using v3.0 beta 11 under Ubuntu.
[s]EDIT
Let me first check the models in my library to make sure it's not caused by something stupid I did.[/s]
Hans