In affiliate marketing context, cookie overwriting refers to the fact that an affiliate ID tied to a cookie is replaced by an other affiliate ID according to the "last cookie counts principle".
Cookie overwriting can be viewed as a misuse of language because the cookie is not really changed, it is only the associated affiliate ID which is changed on the affiliate network’s servers.
Cookie overwriting is a hot topic in affiliate marketing as content affiliates complain that their cookies (in fact the affiliate network’s cookies) are often overwritten by voucher codes, cash back or parasitic affiliates’ ID. Indeed these actors are often the last touch point in the conversion path.
However, some recent studies seem to suggest that cookie overwriting is an overestimated issue as the majority of sales only have one publisher referrer in the path to conversion and take place within 24 hours of the click.
An other Affiliate Window finding was that where cookie overwriting is present, it is most likely to be from a publisher within the same promotional type. Cashback sites are most likely to overwrite other cashback sites rather than true content sites.
An Affiliate Window study made for a telecom sector advertiser in 2012:
Cookie overwriting can be also be "more legitimate" when the cookie is overwritten by an other content affiliate who have a "real" impact on the conversion.
Cookie overwriting is probably a real issue, but the scale of the phenomena is still hard to evaluate with a need for neutral dedicated studies and one may believe it varies by sector of business activity.