i have smoked weed that was definateley poisoned.it was really heavy, had a fucked up taste, made me trip badly and get sick. it was given to us by someone who couldnt stomach it.
never underestimate the possibility of smoking pesticide.
some unscrupulous growers will spray ripe buds with miticides particulary which are pretty rugged chemicals.
i have heard first hand from a major grower round here that he dunked whole massive heads in buckets of double strength kelthane to rid them of mites and make them "saleable".

the only pesticides registered for combustable crops would be the ones used on tobacco. wonder hwy that shits so bad for u?
combusting a clorine based chemical is pretty nassty.
its not common, but it certainly happen far far far to fuckin often.
beware kiddies. grow your own.