It happens when the leftmost of the 7 random barrels is 0.  That barrel will then roll left off the gate at the bottom.  This is why I used an F barrel instead of a 0 under that gate. 
I should note that I used the single stepping in my beta version of Rubicon to track this down.  What are you people waiting for?  You know you want to download it!

For Random----- fixed v2 (nakidex): dyhemom

I used the same principle as jnz

The single-stepping with the 'S' key is definitely a big pro. After I used it the first time, I wondered how I have ever lived without it... I used to use paper storyboard diagrams to visualize why a construction was not working the way I thought it would; now I just single-step through them.
It made me immediately wish for a "step backwards" option as well.

The "fast forward" function is also welcome, although it is not very intuitive that you need to hold down the 'S' key for that. I would have expected that holding down the 'S' key would invoke a "slow motion" function.

Were you able to contact Kevin already about putting this up as the official version on the web-site?