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It means something tried to use memory that it wasn't allocated. For example, you might get this when you have a 4 element array and try to use array_name[4] (numbering starts from 0, so 4 elements = 0,1,2,3)
You probably passed an argument wrong which wasn't interpreted as being wrong, so avisynth crapped out on you. Not all filters have error checking in their arguments. What Kariudo said (he's actually right for once, I'll be damned!)