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Conventional wisdom suggests that bigger switch buffers translate to lower packet loss. However, we have observed in simulations (using ns2) that buffer sizes in the range of interest for optical packet switched networks show unexpected behaviour: larger buffers can cause higher losses for open-loop (real-time) traffic when it multiplexes with closed-loop (TCP) traffic. In this short paper we develop a simplified Markov Chain model that helps explain this anomalous behaviour. The phenomenon observed in this paper can be of serious concern to all-optical packet switch designers and network service providers, who make huge investment in setting up the network infrastructure, but only to realise potentially degraded performance if appropriate care is not taken when dimensioning their router buffer sizes.