I've recently been having some problems when using 'trigger once' when there are multiple instances of an object.
Have a look at This simple capx.
Basically, it's a very simple code that tells the squares to change their opacity when they hit the grey square.
However, you will notice that the very top instance never changes its opacity. If you remove the 'for each', even stranger things happen.
(I know I could just use 'on collision' but this is just for example purposes ;) )
Is this a bug in C2, or just a weird side effect of 'trigger once'?