My son had a farher at a cetain yeshiva on Sunday and although he did not tell me too much about what they asked, he mentioned that the interviewer posed a kashe of R”Y Salanter which the interviewer said he had no good answer for. All eidus must be “efshar l’hazima” i.e. it must be possible for the eidim to be proven zomimim by other testimony. Given that there are a limited number of people in the world who can be eidim, if everyone in the world pairs up into groups to testify, the last group will have no one who can possibly prove them zomimim. If this last group is disqualified because their testimony is not efshar l'hazima (because there is no one else left in the world who can do so), the second to last group becomes the last group, and again, no kosher witnesses are left who can prove them zomimim. Following this chain of regression backwards, we can disqualify all witnesses – so how does this halacha of efshar l’hazima work?
I understand why this is a nice brain teaser to give a 13 year old, but I am not sure why the interviewer himself claimed to be stumped, unless I am missing something. Efshar l’hazima does not mean practically (b’poel) that there has to be some witnesses out there who can make eidim into zomimim; it means that the eidus has to have the *quality* of specifying a date, place, time, etc. which render it susceptible (b’koach) to hazamah.
(One might claim that whether efshar l’hazima means a potential for hazamah or b’poel actually being able to carry out hazamah depends on the two opinions in Tosfos Makkos 2a d”h m’idin – I think the point is debatable, but I’m still happier saying the point depends on two deyos in Tosfos than to leave the question as unresolvable).