  The blockchain technology promises to transform finance, money and even
governments. However, analyses of blockchain applicability and robustness
typically focus on isolated systems whose actors contribute mainly by running
the consensus algorithm. Here, we highlight the importance of considering
trustless platforms within the broader ecosystem that includes social and
communication networks. As an example, we analyse the flash-crash observed on
21st June 2017 in the Ethereum platform and show that a major phenomenon of
social coordination led to a catastrophic cascade of events across several
interconnected systems. We propose the concept of ``emergent centralisation''
to describe situations where a single system becomes critically important for
the functioning of the whole ecosystem, and argue that such situations are
likely to become more and more frequent in interconnected socio-technical
systems. We anticipate that the systemic approach we propose will have
implications for future assessments of trustless systems and call for the
attention of policy-makers on the fragility of our interconnected and rapidly
changing world.
