could it possibly happen that the function accidentally doesn’t use a particularly category at the beginning of the Block/trial set, and is therefore ‘forced’ to use them at the end and crash itself?
When this happens, the function simply generates a new order. This is precisely why it could get stuck into an infinite loop: its a brute-force method, it keeps generating random orders until one matches the specified pattern. As long as your items make it possible to generate a matching pattern, the function will eventually return one.
Jeremy