a) not cleaning their cage, bedding, and litter box frequently enough, including scooping said litter box once or twice a day,
b) feeding their ferret low-quality food, such as Marshall's brand, that contains fish products, which increase the quantity of poops as well as causing more oil production on the ferret's skin,
c) bathing the ferret more than once every couple of months in an attempt to rid it of its natural scent, thereby drying out its skin and causing it to overproduce the oils that cause the stench in the first place.
Most domestic ferrets won't see your small critters as prey unless that's what they are taught - but it's best not to chance it. I tried letting my first ferret out while the bunnies, pigs, and chins were caged, but the bunnies threw a bit of a thumping fit. Then, I tried blocking off half of our living room to let her out in a space away from the other critters. It didn't work too well - she consistently found ways out of my playpen-turned-fence, no matter what I did. Eventually, I moved her in to our computer room/office, which she now only has to share with ferret number two.