But what is a "good technology", what does it mean to "do good", and how closely do these two match? It might seem extremely beneficial to convert or export a problem into something that can be worked with on a blockchain, and then "optimize" what that blockchain object does against some notion of good. Unfortunately, this can sometimes lead to important factors being overlooked or lost, and moreover could cause harm in completely unintended ways. In this talk I will discuss some of the experiences I have had working with blockchain startups, as part of the work I do in the Ethics in Mathematics Project. I will explore how, even with the very best of intentions, mathematically-powered work (such as blockchain tech) that aims to do good and help society might still have undesired, and possibly even harmful, consequences.