The lack of post-deployment visibility into system operation is one of the major challenges in ensuring reliable operation of remotely deployed embedded systems such as wireless sensor nodes. Over the years, many software-based solutions (in the form of debugging tools and protocols) have been proposed for in-situ system monitoring. However, all of them share the trait that the monitoring functionality is implemented as software executing on the same embedded processor that the main application executes on. This is a poor design choice from a reliability perspective. This paper makes the case for a joint hardware-software solution to this problem and advocates the use of a dedicated reliability co-processor that is tasked with monitoring the operation of the embedded system. As an embodiment of this design principle, this paper presents Spi-Snooper, a co-processor augmented hardware platform specifically designed for network monitoring. Spi-Snooper is completely cross-compatible with the Telos wireless sensor nodes from an operational standpoint and is based on a novel hardware architecture that enables transparent snooping of the communication bus between the main processor and the radio of the wireless embedded system. The accompanying software architecture provides a powerful tool for monitoring, logging, and even controlling all the communication that takes place between the main processor and the radio. We present a rigorous evaluation of our prototype and demonstrate its utility using a variety of usage scenarios.