R. 114(1) EPC requires, among other things, that third-party observations state the grounds on which they are based.
In T 189/92 the board held that it was the responsibility of the third party to ensure that the facts and evidence filed were not only unequivocally clear but also as complete as possible, so that they could be handled directly by the investigating instance without doubt and without further inquiry.
In T 908/95, the evidence adduced to show fulfilment of the criteria for establishing public prior use, in particular public availability, did not substantiate or prove such fulfilment to the extent that the allegation could be accepted. The board found that the additional inquiries needed to prove it, which might even include the hearing of witnesses, could only be conducted with the co‑operation of the "third party", but that this was impossible where the person in question was not a party to the proceedings. The third party's observations therefore had to be disregarded (see also T 73/86).
In T 301/95 date: 1997-06-27 (OJ 1997, 519), the opposition division had disregarded third-party observations as to a claim of public prior use, as the alleged facts had not been proven with near certainty.