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Understanding the chemical structure of the solid electrolyte interphase that forms and evolves during lithium-ion battery cycling is critical for advancing battery technology. This complex task often requires the use of postmortem protocols to extract the electrodes in controlled states of charge and prepare them for further characterization and analysis. Over decades of research and optimization, the scientific community has established and shared post-mortem workflow protocols tailored to specific techniques. However, numerous sources of artifacts can disturb this workflow, introducing experimental uncertainties at various stages, from electrode manufacturing to data interpretation. Here we present the results of a round-robin inter-laboratory study using post-mortem X-ray photoemission spectroscopy to characterize the solid electrolyte interphase formed on graphite electrode after cycling in two different electrolytes. Several leading European research teams, expert in battery manufacturing and characterization by X-ray photoemission spectroscopy, participated in a meticulously designed post-mortem workflow. The goal was to identify the sources of consistency and disparity in the results and their impact on the scientific conclusions. Moreover, human-induced bias and errors were quantified throughout key steps, from cell assembly to photoemission core level peak fitting and interpretation. Based on our findings, we offer key recommendations for identifying and minimizing sources of artifacts in the analysis of the solid electrolyte interphase chemical composition. Effectively addressing these challenges is essential for improving both the performance and longevity of batteries.