A modification subsystem suitable for introducing user-defined errors into a designated data unit substantially in real time, so as to facilitate evaluation of the response of a high speed data communications system to errors as such are exemplified in the modified data unit. The modification subsystem includes a modification logic and state machine that communicates with a command stack memory and one or more jamming mode registers. Upon locating and identifying the designated data unit in a data stream, the modification logic and state machine accesses a jam setting stored in a jam mode register and, if it is determined that the modification that corresponds to the jam setting is valid, modifies the designated data unit in accordance with the jam setting. If the proposed modification is invalid, the modification is aborted.