Actually, substance-induced psychosis is a well-documented condition, and it is known to cause schizophrenic episodes in people with underlying disorders, not all of whom will know that they do. More insidious is the way it can mask symptoms of mental illness (the early pangs of which might have lead to cannabis use in the first place) and prevent people from seeking or being provided with the help they need.
http://www.ncbi.nlm.nih.gov/pubmed/17662880
http://www.ncbi.nlm.nih.gov/pubmed/15871146