If I just start up my [default startup file](http://www.pasteall.org/blend/26248) and render an animation, the previous frame is always replaced with a blank transparent checked pattern before the next frame starts rendering.

However, in [this file](http://www.pasteall.org/blend/26233) the next frame is displayed as rendering over the previous frame. 

I have also noticed that in the file linked above some tiles will briefly appear transparent (checkered) before rendering, however others do not.

Why is this?  

Is there some setting I don't know about?

Is it possible I could render animations without anything appearing transparent?