Is This a Bug in the TParallel.For Routine

Is This a Bug in the TParallel.For Routine
I've created a simple demo which mimics rendering a multi layer map.  The demo creates 11 random layers and then merges them together (in the main thread) to form the final image. However, when I render using a normal for loop I get a different image than when I render using the parallel loop (where there is missing data).  See here:

http://screencast.com/t/kdRFJApGxaR

So my questions are:
1.  Am I doing anything stupid here?
2. Can the canvas be used in this form i.e. as part of a parallel for loop
3. How can it be corrected?
4. Or is it just a bug?

Download the code here: http://goo.gl/MPb1ve

Thanks - Steve
http://screencast.com/t/kdRFJApGxaR