The authors analyze how shift errors propagate in a particular run-length limited (RLL) modulation code that is widely used in modern high-density data storage machines. More specifically, the probability that either a one or two byte burst error in the user data will result from an arbitrary channel data shift error is calculated. The authors consider the case in which the spatial recording density, is near the intrinsic resolution of the head/medium interface; here shift errors occur with high probability only for certain worst-case patterns of channel data, i.e., when the channel 'one' under consideration represents a recorded write signal transition that suffers intersymbol interference (ISI). The authors treat the case in which the recorded indicia on the medium are well resolved on playback; here shift errors are equally probable at all channel 'one' locations.>