According to T 657/11, in opposition proceedings mistakes or errors concerning the claims, the description or the drawings of the patent as granted may be removed either by an amendment occasioned by a ground for opposition under Art. 100 EPC or, as far as the mistakes or errors concern texts or drawings which remain unamended, under certain conditions by way of a correction pursuant to R. 139 EPC.

The claims according to the main request before the opposition division and the appellant's initial request in the appeal proceedings differed from the granted claims only in that the wording "NF permeate" in the last process step of claim 6 was replaced by "NF concentrate". The board had communicated that R. 140 EPC was not available to correct the text of a granted patent (G 1/10, OJ 2013, 194). However, the claims according to the appellant's final (sole) request were, as compared to the claims as granted, amended beyond the mere removal of an error, namely by limiting them to those granted as (process) claims 6 to 11. Hence the basis for the decision on the appeal (and, thereby, on the opposition) was no longer that for the decision to grant, which would definitely lose its effect and be replaced by a new decision. In such a situation any (further) amendment of the claims, even if aimed at removing an obvious mistake in the claims as granted, did not constitute a correction of an error in a decision of the EPO within the meaning of R. 140 EPC. As pointed out in G 1/10, it was always open to a patent proprietor to seek to amend his patent during opposition or limitation proceedings and such amendment could remove a perceived error. However, an amendment with the (sole) aim of removing a mistake or error in the claims, but also in the description and drawings of the patent as granted, could not be said to be "occasioned by a ground for opposition under Art. 100 EPC" (R. 80 EPC). It followed that such mistakes or errors – here the expression "[nanofiltration] permeate"- in the unamended part of the text could only be removed by a correction pursuant to R. 139 EPC, which provision and the specific requirements defined therein applied independently of R. 80 EPC. The proposed correction was in accordance with those requirements.

In T 488/13 the board considered that a printing error, which could not be imputed to the patentee, should normally be correctable at any time. In the case in hand the proposed amendment effectively corrected such a printing error, so that the amendment was appropriate and necessary.