Ive heard of it yes, sure there was a guy on the eboka forums that had a negative experience. Couldnt see anything when I had a quick look. Ive also heard of people having negative ayahuasca experiences and being depressed for months afterwards, there was a thread on the ayahuasca forums, again I cant find it sorry but Ive read of such things yes. Dig around here, eboka forums and ayahuasca forums and you will find reports.