I read on STEEN, RG. Retractions in the scientific literature: is the incidence of research fraud increasing?. Journal Of Medical Ethics. England, 37, 4, 249-253, Apr. 2011. ISSN: 1473-4257. that:
Journals do not do a careful job of alerting the naïve reader to a retraction notice (table 2). The most common way to alert readers about a retraction is with a watermark on the pdf (41.1% of retracted papers). A total of 149 papers were given such a watermark and noted as retracted at the website as well. Among 305 watermarked papers, 48.9% were also retracted at the website; of 248 papers retracted at the website, 60.1% were also watermarked. However, 31.8% of retracted papers were not noted as retracted in any way at all so the naïve reader would not be alerted to the fact that retraction had happened.
How comes 31.8% of retracted papers are not noted as retracted?