Deviations from the ideal single-file behavior in actual systems, such as zeolitic host-guest systems with a one-dimensional channel structure, may occur if (i) there is a possibility of mutual passages of the diffusants and (ii) the influence of the file boundary on the movement of the diffusants cannot be neglected. The conditions under which such deviations may become relevant are investigated both by analytical methods and molecular dynamics simulations. The consequences of these deviations for experimental studies of single-file diffusion are discussed.