I think the worst thing a dealer has ever done to me is charge me an excessive price, and/or sell me mediocre drugs without forewarning. Beyond that, just the usual "dealer-time"
In the one case I can think of where it was both, in fairness, he thought it was perfectly good (the particular batch of "ketamine" - we're still never sure which batches are actual K, as my friends and I see enough variation in K that we can't explain it by just a continuum between the two isomers; we conclude that a significant proportion of "ketamine" is actually closely related drugs, which we refer to as ketaminoids, drugs that have noticeable qualitative differences, yet fill the same drug-role). Actually I thought it was fine too, not my ideal type of K, but I'd still do it - most of my friends didn't like it at all. And while he definitely overcharged me, apparently k just generally just goes for oddly high prices in the circle he's in anyway; others in that circle felt I was only mildly overcharged.
Never got straight up bunk (except from new RC vendor a couple of times - the stuff was potent and fun, but not what was advertised in most cases; in one or two, when it was one of the first batches of something new, it was really impure, but the early batches of a lot of new RCs are like that - but RC vendors aren't dealers IMO)
Most of my other regular suppliers are straight with me about their price and what markup they charge, and I consider the markup fair, and it's all good. One time during a shortage of a specific drug, they got a really crappy batch, told me it sucked, and gave it to me for a really low price that was fair in light of that.
