In order for a correction in the description, the claims or the drawings to be allowable under R. 139, second sentence, EPC, the boards apply a two-step approach. It must be established (i) that it is obvious that an error is in fact present in the document filed with the EPO, the incorrect information having to be objectively recognisable by the skilled person using common general knowledge (G 3/89, OJ 1993, 117 and G 11/91, OJ 1993, 125, point 5 of the Reasons), and (ii) that the correction of the error is obvious in the sense that it is immediately evident that nothing else would have been intended than what is offered as the correction (G 3/89 and G 11/91, point 6 of the Reasons).

Since the prohibition of extension under Art. 123(2) EPC also applies to a correction under R. 139, second sentence, EPC (G 3/89 and G 11/91, point 1.4 of the Reasons), the parts of a European patent application or of a European patent relating to the disclosure (the description, claims and drawings) may be corrected under R. 139, second sentence, EPC only within the limits of what a skilled person would derive directly and unambiguously, using common general knowledge and seen objectively and relative to the date of filing, from the whole of these documents as filed (G 3/89 and G 11/91, point 3 of the Reasons).