We investigate, experimentally and theoretically, the influence of disordered defects on the transmission properties of optical delay lines based on coupled cavity waveguides (CCW's). Also, the modification of transmission for a line-defect waveguide in a two-dimensional (2D) photonic crystal (PC) upon bending is examined by numerical simulation. We reveal that the effect of waveguide bending on the transmission properties of PC circuits is very similar to that of disordered defects on the transmission properties of CCW's. It is shown that the broad impurity band of a one-dimensional CCW will evolve into sharp defect modes upon increasing the disorder. Using a symmetric Mach-Zehnder structure formed in a 2D PC as an example, we show that a PC circuit containing multiple bends and branches of different properties generally gives rise to sharp resonant modes whose linewidth is inversely proportional to the size of the PC circuit. This might be an intrinsic problem in realizing PC circuits.