I'm sure it has happened at least once, but I suspect that the DEA gives it press coverage way out of proportion to its incidence.
As an adolescent of 12 or so, I remember watching a D.A.R.E. video in school where the fictional character tried marijuana for the first time, but it was secretly laced with cocaine and the fictional character had some kind of unspecified heart condition and hyper-sensitivity to cocaine that caused his first puff of pot to kill him. Remember kids, your first puff of pot can kill you!
They also told us that sometimes PCP is laced with rat poison.
Even as a 12 year-old, the implied frequency of such craziness beggared belief. It would have been more believable if they would have warned us that we shouldn't try hard liquor because it might be tainted with methanol. That at least happens with some frequency back in the Old Country, according to a Bulgarian friend.