I'm going to share my experience with cannabis that was likely laced.
I was living in a city in arizona at the time and a friend of my neighbors (they lived in the guest house in the back yard) had come by to smoke bong loads. He had brought probably just over an 8th of "Bobby Brown" to smoke. I was suspicious of him, and I wasn't the only one. He was a short mexican guy who no one really knew and he had brought the weed wrapped up in a paper towel. When the bong was passed to him he politely declined, indicating that the smoke was for us, and he could get us more for 200 per pound. At that point I was starting to get kind of sketched out, but accepted a hit against my best judgement. The hit didn't taste great, but I hadn't expected much from what was obviously low quality mexican shwag. I declined every hit afterward, and steadily got more and more drowsy. Everyone else continued to smoke the rest of it. I picked up a bud to examine it before it was gone and noticed that the brown color appeared to be caused by a powder, or spray on the surface of the bud. Everyone became extremely lethargic and eventually went to sleep. I was not nearly as effected, becoming relaxed, but not terribly inebriated. The next morning everyone woke up feeling hung over. Mine wasn't as bad as everyone else, but I still felt lethargic. We all agreed that it had been laced with some sort of opioid, possibly heroin, but more likely a pharmacutical that had been disolved, sprayed, and dried on the surface of the bud. I should note that although I don't have any other experience with opiates, two of my roomates had experience with pills and with smoking heroin.
I would like to end this post by saying that yes, people do lace weed. Usually it is very low quality ganja that they have been unable to move otherwise. It is also usually done by people who have access to a plentiful supply of some sort of cheap narcotic. It also seems to be done more often near the boarder, since pounds and pounds of low quality cheap grass are available in mexico, as well as cheap narcotics. Someone can take $100 worth of cannabis and double the street sale value in the states simply by adding $20 worth of something else. I have also heard of coke laced stash being sold in the Los Angeles area, but this is usually sold as such. If you have any doubts about the source, it is probably safer to pass and rely on your own stash from your own dealer. Buy high quality sensamilla bud, as it is less likely to be tampered with. If the surface appears powdery rub at it lightly with your fingertip and watch for powder. Kief has a very different appearence from coke, heroin, or any other addition to the cannabis.