Most fentanyl in non-opiates (such as cocaine) is the result of accidental contamination, not intentional cutting. It happens late in the supply chain: a dealer

Most fentanyl in non-opiates (such as cocaine) is the result of accidental contamination, not intentional cutting. It happens late in the supply chain: a dealer sells two different drugs, uses the same scale for both, and doesn't completely sanitize them in between, so you get a few remaining "crumbs" or residue of fentanyl accidentally mixed in. This is an issue with fentanyl because it's so potent that a small amount which would be a non-issue for opiate users can easily kill a person with no tolerance for the drug. It's also dangerous because, when mixed with stimulants like cocaine, the overdose doesn't necessarily happen immediately - the stimulants will briefly mask the effects of the opiates, raising the heart rate, but then they'll wear off and the opiates will suppress breathing to the point of death. Because the effects aren't always immediate, the drug user (or those around them) don't connect the person's behavior to the drug use, so they don't always recognize it as an opiate overdose requiring immediate emergency attention and Narcan. Source: used to work for a nonprofit that conducted harm reduction education training