Hey now! Although I agree that it is probably a rare scenario... I do have two examples to the contrary:
Back in highschool, after our dro dealer had moved away, my friend and I once bought some schwag that had a thin strip of what looked like blotter paper in the bag with it. We were quite upset, because he had assumed that the paper was included in the weight, and this guy's weed was usually dirt anyway. The weed itself looked even worse than usual, but, being the reckless highschoolers that we were, we indulged. A single hitter got us all higher than the dro we had been used to, but I remember a distinct burning sensation in the eyes and tongue. The taste is a bit harder to remember all these years later, but it reminded me of that dentist's office smell. Anyway, after a second bowl I became incredibly uncomfortable. My vision started to tunnel and it was as if the contrast had been turned up. I guess you would call it "seeing stars", but everything seemed to become pixelated and oscillated in and out of brightness and clarity. I heard a flanging sound reminiscent of nitrous oxide, and slowly the previously-mentioned pixels replaced everything in my field of vision. Everyone and everything was made up from these, but they were moving at different rates, so solid objects (and especially people) slowly lost their continuity. Watching this unfold quickly became a cumbersome task, so I attempted an escape by closing my eyes. This provided temporary alleviation, but soon the light patterns on the backs of my eyelids took amorphous forms of eyes and teeth. (It should be noted that I have never had a negative experience prior or since and that these shapes did not appear scary to me; it was more of an overwhelming, or even annoying experience. I wouldn't put the potency of this experience beyond the limits of THC; I have had near religious experiences from the stuff, but this was quite distinct in its effects.) Still uncomfortable with my current state, I opened my eyes again only to find the pixelation resuming. From there on out, I oscillated between opening by eyes and holding them them shut as each became difficult to bear. Needless to say, at the end of the experience I was entirely worn out, and a splitting headache and deep red eyes plagued me the rest of the day. Now I, too, would chalk this up to my being young, inexperienced and psyching myself out, were it not for the next day! You see, we kept that tiny strip of blotter paper and the next day when a third friend didn't believe us so we told him to smoke part of the strip of paper. He put it into a pipe, and after a single hit, he told us he felt like he had taken a pull from a nitrous balloon. We watched his skin lose its pallor as his eyes became immediately red and soon he was talking about tongue numbness. I do not claim to know what was in our weed that day, (the other two still insist PCP) but I would certainly call that "laced".
My other encounter was back in my distribution days. One shipment I received had not been cured properly and was not quite up to par with what my clientele were expecting, so it had gained a nickname: "the wet beast". After being unable to move it for over a week, I began to worry. Luckily for me, JWH-018 and JWH-200 had just hit the scene in my area and damn were they cheap! I dissolved a gram of each into alcohol and sprayed them onto an ounce of weed (well 2oz, a gram for each). I let several people try it out, and they greatly preferred it to the unlaced beasters. The JWH-200 blend was my favorite, but everyone else preferred the 018 and started calling it "the mad beast", so I mixed up the rest with that and ended up getting rid of all of it all in time for the next shipment which was back to par. Let it be known that I informed all parties of exactly what they were smoking and was not trying to trick anyone! Though I cannot speak for those down the chain..