^very good point on edit #2 tacodude. OP, if you're not tolerant, and pretty highly tolerant at that, it's not difficult to OD off of good dope, cut or not.
And yeah, not all heroin is really processed/purified all the way, especially black tar (which is essentially just opium crudely processed into diacetylmorphine but still contains other opiate alkaloids, definitely codeine).
Also, a small amount of codeine is metabolized into morphine, so that would explain that. I believe that when diacetylmorphine is metabolized, morphine is a byproduct as well, but I'm not totally sure about that so please correct me if I'm wrong.
As as for the stim-related cuts, like tacodude said, don't be surprised. At this point, I'd be surprised to find coke that isn't somewhat sketchy. And if you were speedballing, that could also account for your OD.
Well said TD, never underestimate your dope.