having render texture problems... uncertain whether it's my system or unity

Hi, I'm just beginning to use render textures in a scene because I'd like to have visual fx affect only certain objects in the scene.

My render problem can be seen here: http://imgur.com/7RwQ1

What is happening is clearly a case of feedback (not clearing the render buffer?), but also the fact that somehow the image of my project heirarchy somehow also gets put into the render texture.

Is there something I don't know about render textures, or is this a unity problem, or might it be my particular system?

Machine specs are:

OSX 10.7.5 2 x 2.4 GHz Quad-Core Intel Xeon 20 GB 1066 MHz DDR3 ECC ATI Radeon HD 5770 1024 MB