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The occurrence of bit-error-rate floors due to coherent heterodyne crosstalk in photonic space switches is studied. This source of errors occurs when a particular switch is used to route signals of nominally the same wavelength in a WDM network, DFB lasers are run at multigigabit rates, and wavelength referencing is held to a relatively tight tolerance to facilitate wavelength demultiplexing. In this letter we compare the dependence of switch architecture and size on the rate of coherently induced bit errors. Three regions of interest are identified: (1) random uniformly distributed laser phases with binomially distributed bit statistics in interfering channels, (2) random uniformly distributed laser phases with ones in interfering channels, and (3) worst-case laser phases with ones in interfering channels. Critical crosstalk values are identified for four different switch architectures and two different switch sizes in order to avoid coherently induced errors and therefore BER floors due to this mechanism. The minimum crosstalk required to avoid coherent errors can vary from -10 dB to -42 dB depending on the switch architecture, size, and requirement for short-term BER stability.