The boards of appeal have drawn a distinction between the hearing of witnesses and the hearing of opinions by experts: a witness is put forward to substantiate facts of which he/she has personal knowledge. In T 311/01 the appellant (opponent) offered witness testimony on the skilled person's knowledge and understanding of the cited prior art. However, the testimony was offered as evidence not of specific facts but of the knowledge and ideas of skilled persons in the technical field concerned, so that the appellant was in fact offering experts, not witnesses. Regarding itself expert enough with regard to the features and advantages described in the prior art, the board refused to hear the proposed "witnesses" (see also T 1511/06 and T 32/10).

In T 480/11 the subject on which the proposed technical expert Mr J. intended to speak according to the appellant's request was not simply a technical issue but an event in the past, namely the performance of experiments and the results obtained thereby, which had taken place at the appellant's laboratory. So the appellant's request was actually directed to hearing Mr J. as a witness rather than as a technical expert. The board decided not to hear Mr J.

The board in T 1676/08 (case with five members) stated that assessment in a case where the board including three technically qualified members assesses technical facts in the light of patent law and considers itself expert enough to decide upon a matter without technical assistance from an expert within the meaning of Art. 117(1)(e) EPC, is a matter for the board and not for a technical expert. Such assessment does not mean that any member of the board becomes a witness or an expert.