In opinion G 3/89 (OJ 1993, 117) and decision G 11/91 (OJ 1993, 125) the Enlarged Board held that corrections under R. 88, second sentence, EPC 1973 (now R. 139, second sentence, EPC) were special cases of an amendment within the meaning of Art. 123 EPC and fell under the prohibition of extension laid down in Art. 123(2) EPC.
The parts of a European application or patent relating to the disclosure (description, claims and drawings) can be corrected only within the limits of what the skilled person would derive directly and unambiguously, using common knowledge and seen objectively and relative to the date of filing, from the whole of these documents as originally filed. Such a correction was of a strictly declaratory nature and thus did not infringe the prohibition of extension under Art. 123(2) EPC 1973.
For case law on these issues prior to G 3/89 and G 11/91, see the summary of facts and submissions of those decisions, and "Case Law of the Boards of Appeal", 7th ed. 2013, II.E.4.1. For a summary of the findings in G 3/89 and G 11/91, see also G 2/95 (OJ 1996, 555, point 2 of the Reasons).