Art. 40 CISG
The seller is not entitled to rely on the provisions of articles 38 and 39 if the lack of conformity relates to facts of which he knew or could not have been unaware and which he did not disclose to the buyer.
The Secretariat Commentary on Art. 38 of the 1978 New York Draft, predecessor to Art. 40 CISG, gives insight into the drafters' unterstanding on the eve of the 1980 Vienna Diplomatic Conference.
CISG-AC Opinion regarding Art. 40 CISG
Overview of case law on Art. 40 CISG: UNCITRAL Digest
The UNCITRAL Digest on the CISG can be used as a starting point to discover the relevant case law on this article.