Information presented after a witnessed event may become incorporated into a person's memory, and either supplement or alter the original memory. One consequence is a distorted recollection of the event. Many variables influence the likelihood that memory is distorted by post-event suggestion, including (1) the violence of the event itself, (2) the time intervals between the event, post-event information, and final test, (3) the syntactic form of the post-event information, and (4) the presence of warnings cautioning about the possibility of post-event suggestion, and the timing of those warnings. A common principle tying these findings together is the notion of `detection of discrepancies'. Recollection change is enhanced to the extent that a person does not detect discrepancies between the original memories and the post-event information at the time that the latter is being processed.