An important characteristic of tunable lasers, that may affect system performance, is that they can generate light at various wavelengths as they tune between different wavelength channels. These spurious components could cause severe cross-channel interference if the tunable laser is used in a WDM network. In this paper, we investigate the effect of such interfering signals, generated by up to three tunable lasers during a switching event, on the BER of a data channel transmitted on the same wavelength as the spurious components. The results show that it is vital to attenuate the output of tunable lasers during their wavelength transition in order to prevent system performance degradation.