In G 3/89 and G 11/91 the Enlarged Board held that the parts of a European patent application as filed which relate to the disclosure must further allow a skilled person – using the common general knowledge on the date of filing – directly and unequivocally to ascertain the precise content of the information the person making the request actually meant to give, instead of the incorrect particulars, on the date of filing or when making an amendment under Art. 123 EPC, so that, for said skilled person, "it is immediately evident that nothing else would have been intended than what is offered as the correction" (R. 139, second sentence, EPC). However, if there is any doubt that nothing else would have been intended than what is offered as the correction, a correction cannot be made.

Already prior to G 3/89 and G 11/91, the boards had allowed corrections under R. 88, second sentence, EPC 1973 (now R. 139, second sentence, EPC) provided it was immediately evident to the skilled person that an error had occurred and how it should be corrected (see "Case Law of the Boards of Appeal", 7th ed. 2013, II.E.4.2).

In J 5/06 the board, referring to T 158/89 (which had not accepted correction in case of two equally plausible alternatives for a percentage range of a component), held that establishing that a suggested set of documents was a probable and suitable replacement did not amount to establishing that nothing else would have been intended. This latter threshold implied that there was only one single plausible replacement – the one which the skilled person would have deduced from those parts of the application which made up the disclosure of the invention. For the case in hand the board concluded that, to the contrary, a complete exchange of the application documents would quite obviously open the door to a plethora of plausible replacements. See also J 16/13, also relating to replacement of application documents; T 15/09 and T 846/16, both relating to correction of a feature in the claims at issue.

In T 955/92 the reasons given by the appellant to show that only the requested correction could have been intended were not based on the level of general knowledge on the date of filing. In order to arrive at the conclusion that the requested correction was the only physically meaningful one, experiments had had to be performed which needed more than common general knowledge, and the results thereof would not have been available on the date of filing. The request for correction was therefore refused.

In T 438/99 the board held that the fact that a term or phrase could not be interpreted or construed because it was unresolvably ambiguous did not necessarily mean that its deletion was a permissible amendment under Art. 123(2) EPC 1973. There remained a residual clear meaning in the ambiguous term, e.g., as in this case, that a specific direction was taught, and suppressing that fact resulted in a different technical teaching. Therefore, the second condition of R. 88 EPC 1973 (immediately evident that nothing else would have been intended than what is offered as the correction) was not met by the offered correction of deleting the feature without replacement.

T 1728/07 concerns the correction of an error in a structural formula representing oxazoline derivatives. The board recalled that according to R. 139 EPC, second sentence, it must be immediately apparent to the skilled person that (i) an error has occurred and (ii) how it should be corrected. With respect to requirement (ii), it must be decided whether the corrected feature is directly and unambiguously derivable from the content of the application as originally filed taken as a whole. This requirement was found by the board to be fulfilled.

In T 423/08 there was an obvious mistake in the original wording of one of the claims, in that either "gemäß" (according to) or "vor" (before) was superfluous. Although deleting "gemäß" made sense when considered in isolation, it was incompatible with the wording of claim 1. Deleting "vor" was thus not open to objection under Art. 100(c) EPC.

In T 1508/08, the board concluded that the second condition for allowability of a correction under R. 139 EPC was not met. In its view, not only did the correction fail to solve the existing problems of a lack of clarity, but it actually made things even less clear. That was one reason why the skilled person would not have contemplated the correction made by the appellant proprietor. Even if it were assumed in the appellant's favour that the skilled person would have contemplated this correction, it was not the only one possible, but rather one of at least three conceivable corrections.

In T 455/09 the board held that in the case at issue the skilled person could not exclude one of two options with certainty, so that it was not directly and unambiguously evident that nothing else could have been intended other than the proposed correction. For another case where other corrections were equally possible, see T 923/13.

In T 2303/10 the board held that it would be obvious to the skilled person that the first of the two alternative corrections mentioned by the examining division was totally incompatible with what was disclosed in the application as originally filed, whereas the second was technically feasible and entirely consistent with the disclosure and therefore allowable.

T 163/13 concerned the correction of "72EF" to "72°F". The board held that the only possible correction having a technical meaning in the present context was the temperature, as it was an essential parameter for the test in question. Consequently, the skilled person would immediately consider that "F" meant "Fahrenheit" and make the correction accordingly.

In T 657/11 the board held that the requested correction was obvious. It could not accept the argument of the respondent that the correction was not obvious because the wrong wording of the claim was also to be found in one passage of the description as filed. In fact, apart from this one passage, the rest of the description, the working examples and the figures were in line with the perceived correction. The board also excluded another possible correction because, although theoretically and technically possible, the skilled person would immediately reject it.

In T 141/14 the board held that the amendment of the feature "vanadium" to "vanadium oxide" did not meet the requirements of R. 139 EPC because the proposed correction was only one of several options which would occur to the skilled person. As long as there was at least one further possibility of correction, the criterion within R. 139 EPC that "the correction must be obvious in the sense that nothing else would have been intended than what is offered as the correction" was not fulfilled.

In T 606/90 the board allowed a correction pursuant to R. 88, second sentence, EPC 1973, in the opposition proceedings although the correction was not so obvious from the published text of the patent that it was immediately evident that nothing other than the proposed correction could have been intended. The applicant had filed typed amendments to the claims and the description and had also submitted the "working document" with the handwritten corrections for the dossier. The typed version of claim 1 omitted the detail which was now the subject of a request for correction pursuant to R. 88 EPC 1973; the same detail was present in the corresponding passage in the description. When the text was being prepared for publication of the patent, the detail in question was also omitted from the description owing to an error by the EPO. The board concluded that the omission in the description should not be taken into account when considering the admissibility of the correction. The reader of the patent would immediately have noticed the need for correction of the error in claim 1 if a transcription error had not likewise been committed by the EPO.