Moments later, some very intoxicated tourists dive into what they think is cocaine. They do a line each, don't get the cocaine feeling that perhaps one or two of them vaguely remember, but they assume that they just got badly cut drugs, so they dive in and do more.
Snorting heroin (especially if you think it is cocaine) is a surefire recipe to die because it tends to take a little longer to really kick in. And, because of its toxicity, there isn't a lot of room between getting high and dying.
I'd be surprised if that didn't happen all the time with heroin too, though if heroin doesn't provide a numbing feeling then there's no motivation to specifically add a cheap numbing drug.. but numbing could well be an accidental side-affect of something that was added to dilute the % of heroin in each gram sold?
Could be. As [boefs mentions on Reddit](http://www.reddit.com/r/thenetherlands/comments/2npjx5/just_...), selling Heroin in the Netherlands is really hard.
The two most likely scenario's at this moment is that either someone got his hands on cheap Heroin and wanted to make a quick sell or that someone was ripped and sold a large quantity of Heroin and now wants to make some money off that. Either way, in the Netherlands you apparently hardly get rid of heroin, whereas coke is sold for much higher prices much easier.
This suggests that there is some motivation to pass heroine off as cocaine, rather than this being a supply-chain mixup. But then why sell heroine as the substitute? It seems like the street value of heroine would still be higher than other substances (say, talc with the local anesthetic). If you're dealing, why not just try and pass that off, increase your profit margin, and not invite criminal investigation into the deaths of your customers?
Or is the presumption here that some people are dealing with the purpose of causing harm?