Molds can sometimes grow to an impressive size. A story has been making the rounds in Montreal about a large piece of mold being found in a sample of Oasis juice. You would think from the press coverage that such a thing has never been seen before. Another sample was found last March in a carton of Minute Maid orange juice in Britain. In that case a little girl got sick from drinking the juice. In Montreal it was just yuuuck.
There are many species of mold and their spores are found everywhere. A sugary environment is very conducive to growth. Some molds are dangerous because they produce a variety of toxic metabolites. Aflatoxins produced by the Aspergillus species that can contaminate peanuts are an example. But the mould used to produce blue cheese is obviously safe. In fact it is of the Penicillium species which of course also gives us penicillin.
How the juices got contaminated is hard to say. All it takes is a tiny hole in the package for some spores to sneak in. That could happen from jostling during transport or a faulty seal and of course contamination after the package is opened is also possible. Considering the amount of packaged juice consumed in the world this is a rare event. But rare events do occur. Molds that grow in damp areas like bathrooms, basements and kitchens are a bigger problem and can cause watery eyes, breathing problems, rashes and sneezing. Contact with a dilute bleach solution kills surface molds.