Retraction Watch is a blog that reports on retractions of academic articles.
A retraction is a statement in an academic journal that an article previously published in the journal is not valid after all. Usually this is because of plagiarism, fraud or serious errors. For example, in 2010 The Lancet retracted a notorious paper by Andrew Wakefield and others (published 12 years earlier) that linked the MMR vaccine to autism.
The referencing tool Zotero now checks the Retraction Watch database to ensure you are not using a retracted source in your research.