Camera "don't clear" flag not working

I’m trying to use the camera’s “Don’t Clear” flag to get a smearing effect (when an object moves it leaves “copies” of itself behind). I’ve tried several things but can’t get the result I want.

I started with an empty scene and just created a plane and a simple script to move it. But when I run the project the plane moves without leaving copies behind. I’ve tried placing another plane behind it, and also tried using a cube instead of a plane, but I get the same results. I also tried moving the camera instead of the plane.

The camera clear flag is set to “Don’t clear”, and I tried “Depth only” also since I read this gives the same smearing effect. I tried orthographic and perspective projection. Everything else is just the default.

Am I doing something wrong? Am I not understanding how the “Don’t clear” flag is supposed to work?

So not exactly an answer, but more of a work around. I was able to get this to work using Unity 4.0. I was originally testing using Unity 3.5.6f4, Pro on a Mac and free on a PC, and neither worked. When I upgraded my PC to Unity 4.0 it worked as expected (leaving after images behind when the object was moved). I’m not sure why it wasn’t working in 3.5.