In T 17/91 the board took the view that an assertion of public prior use based on the opponent's own activities which is submitted after the expiry of the opposition period and in the absence of good reasons for the delay represented an abuse of the proceedings and a breach of the principle of "good faith" which all parties are expected to observe. As soon as evidence was in the possession of the opponent and it was recognisable that it could be highly relevant to the validity of the patent it should be submitted in the proceedings.
However in T 1019/92 the board held that the fact that an opponent submitted prior art material originating from itself after the end of the opposition period did not constitute an abuse of the proceedings in the absence of evidence that this was done deliberately for tactical reasons. Likewise, in T 481/00 the board considered that the maintenance of a prima facie clearly invalid patent as the legal consequence of the late submission of a clearly novelty-destroying prior use allegation in opposition proceedings, albeit still submitted in time to guarantee the patentee's right to be heard, would in the specific circumstances of its case amount to an inappropriate sanction without a sufficient legal basis in the EPC or in the general principles of procedural law applicable under Art. 125 EPC.
According to the board in T 1955/13, there were good reasons not to treat an opponent's late-filed allegation of its own public prior use differently for the purposes of admissibility depending on whether it admitted to having withheld the alleged prior art deliberately or claimed simply not to have looked for it earlier. If the lateness of its prior-art search could not be explained by procedural developments that would prompt a reasonable person to search in a particular direction for the first time, the opposition division would not be committing any error in law in applying the case law on abusive withholding of prior art when it came to deciding whether to admit its allegation.
In T 154/95 the board ruled that in opposition or appeal proceedings it was basically irrelevant how an opponent came across documents or other evidence made available to the public. Thus there was nothing to stop an opponent from citing a prior use invoked in the same case by another opponent whose opposition was inadmissible because it had been filed after expiry of the opposition period.