In T 515/05 the appellant had based its opposition only on the grounds of Art. 100(a) EPC 1973. In the summons to the oral proceedings the opposition division itself introduced a further ground for opposition under Art. 100(b) EPC 1973. At the beginning of the oral proceedings the chairman of the opposition division stated that no discussion of the ground of opposition under Art. 100(b) EPC 1973 would take place because it had not been sufficiently substantiated by the opponent. Denying the appellant the opportunity to comment on this ground, albeit introduced by the opposition division itself, was considered a substantial procedural violation. The fact that the appellant did not file written arguments in response to the summons to attend the oral proceedings did not deprive it of its right to be heard. The appellant was entitled to expect that it would still have an opportunity to comment on this new ground during the oral proceedings.