Before a correction can be made under R. 139, second sentence, EPC it has to be established what a skilled person would derive, on the date of filing, from the parts of the European patent application relating to the disclosure. As a result of the prohibition of extension under Art. 123(2) EPC, documents other than the description, claims and drawings may only be used insofar as they are used for proving the common general knowledge on the date of filing. On the other hand, documents even if they were filed together with the European patent application, such as priority documents and the abstract may not be used. Documents not belonging to the parts of a European patent application relating to the disclosure could, under certain circumstances, be included partially or wholly in the disclosure by means of reference. Evidence of what constituted common general knowledge on that date could be furnished in any suitable form (G 3/89 and G 11/91).

In T 1008/99 the board held that, for the purposes of R. 88 EPC 1973, the error had to be apparent from the divisional application itself, and the parent application could not be used to demonstrate that the error was obvious. Even if it was apparent from the filed description, claims and drawings that they did not belong together, it was not immediately clear from the divisional application itself which of these parts was incorrect.