Software-defined networks (SDNs) only logically centralize the control plane. In reality, SDN controllers are distributed entities, which may exhibit different behavior on event triggers. We identify several classes of faults that afflict an SDN controller cluster and demonstrate them on two enterprise SDN controllers, ONOS and OpenDaylight. We present JURY, a system to validate controller activities in a clustered SDN deployment, involving topological and forwarding state, without imposing any restrictions on the controller behavior. Our evaluation shows that JURY requires minimal changes to the SDN controllers for deployment, and is capable of validating controller actions in near real time with low performance overheads.