Microservice is an approach to software development, in which an application is designed and constructed to maximise the benefits of modularisation. This approach improves testing of applications, where modularization can limit the prorogation of bugs and facilitates their detection. Though the microservices paradigm has the potentials to ease the automation of bugs detection and fixing, the process is still less understood by the microservices community. To bridge this gap and accelerate such an understanding, we extracted posts from Stack Overflow to identify the most commonly discussed issues in microservice development and testing and we have categorised the concerns. Our results indicate that (i) missing parameters and operations are the most common concerns in service routing; (ii) wrong versions of libraries, annotations, protocols, and clusters appear as the main concerns on service discovery; (iii) the absence of authorisation operation and web tokens exposed for a long time are the main concerns related to service authentication and authorisation; and (iv) the absence of configuration parameters for cache and inadequate patterns for long running transactions emerge as trending concerns in service invocation. We analyse our findings and provide suggestion for future research.