Most people who use E-cigarettes may not realize that the cartridges (even if empty) have the potential to poison a pet, if ingested or chewed on. This would most likely apply to puppies and dogs, but would be especially dangerous to kittens who chew on everything. Cannabis cartridges may have the same potential.
This article on POC http://pictures-of-cats.org/vets-issue-e-cigarette-alert-after-rise-in-pet-poisonings.html speaks to the harmful effects on pets, but I suspect it would also apply to toddlers or young children that might put these in their mouth while teething, or out of curiosity.