Concluding, in this work we have presented the first evidence of crossover-driven side reactions in aqueous organic redox flow batteries. These side reactions increase the rate of degradation of ferrocyanide catholytes via interactions between ferricyanide and anolyte material. Furthermore, simulated-crossover experiments have been developed, which can be used to study these side reactions outside of the battery environment. Challenges in developing suitable organic catholytes mean that ferrocyanide is still heavily relied on in the field. Though these batteries are currently used on a small scale in industry, their long-term stability needs to be established before they are suitable for large-scale grid-level energy storage. The evidence presented in this report suggests that the anolyte chosen significantly influences the stability of the ferrocyanide electrolyte, and hence the lifetime of the battery. It is also possible that similar processes could occur in similar, organometallic catholytes, which are emerging as promising replacement for ferrocyanide. Further work is needed to determine the exact mechanisms contributing to the crossover-driven side reactions reported herein. However, this work reveals new important design rules for anolyte development. Furthermore, it also shows that standard ex situ measurements could lead to an underestimation of crossover in ferrocyanide-based batteries if these crossover-driven side reactions are ignored. Our simple ex situ NMR simulated-crossover experiments can identify when these side reactions are occurring, so that unfavourable anolyte-catholyte combinations can be avoided. These insights not only enable more accurate crossover quantification but will also assist the development of longer-lasting ferrocyanide-based redox flow batteries.