Resource-constrained embedded systems such as wireless sensor networks are becoming increasingly sought-after in a range of critical sensing applications. Hardware for such systems is typically developed as a general tool, intended for research and flexibility. These systems often have unexpected limitations and sources of error when being implemented for specific applications. We investigate via measurement and simulation the output of the onboard clock of a Crossbow MICAz testbed, comprising a quartz oscillator accessed via a combination of hardware and software. We show that the clock output available to the user suffers a number of instabilities and errors. Using a simple software simulation of the system based on a series of nested loops, we identify the source of each component of the error, finding that there is a 7.5 × 10−6 probability that a given oscillation from the governing crystal will be miscounted, resulting in frequency jitter over a 60 µHz range.