cellio | daf bit: Nazir 57

The mishna teaches: if someone says to two nazarites "I saw one of you (ritually) defiled, but I do not know which of you it was", it is as if both have been (so their naziriteships have been interrupted and they both bring korbanot (sacrifices)). The gemara objects: in other cases we require two witnesses, but here a single witness is sufficient? Rabbah ben Huna says: this means the witness saw a source of defilement thrown between them, but was standing far enough away to not see clearly. (57a)

Rabbah seems to take it as given that somebody must have been defiled in that case. I'm not sure of all the parameters here, assuming this happened outdoors (so there are no issues of being in the same tent, etc). Is the idea that everyone within a certain distance is affected, or is it that really only the nearer one should be but we don't know so affect both to be safe?

(Today's daf is 56, but I had trouble drawing something appropriate from that page.)