David and I spoke about this on the phone for a bit yesterday and it is always a shame to see someones rocket come down in a flaming heap. As I always tell people at QRS launches, if something is not assembled correctly, the flyer will end up disappointed, however for the spectators, it generally will be VERY KEWL and look SPECTACULAR!!
Now, for the people who joined the forum in the last couple of years may not have seen the following thread, however I find it interesting that some people commenting here, commented on it way back in 2009 when young (at the time) Alex assembled his O-Rings back to front. This looked unreal, however was a costly exercise indeed. Only days after, Aerotech published a new version of the O-ring chart which is certainly an invaluable tool in every rocketry toolbox.http://ausrocketry.com/forum/viewtopic.php?f=8&t=1909&p=24893&hilit=ring+spacer+guide#p24893
A quick rule of thumb, If you have different size O-Rings, the one that fits snuggly on the nozzle is generally the one that goes there! Further to this, if a part is missing, DO NOT CONTINUE!! All parts are required if it says so.