Dear Rabbi Kornfeld,
One of the examples cited today (Daf 4) was of a needle that had rusted and was no longer tamei because it was no longer usable. (The other possibility is that it was broken). We treat this needle as it exists in its present state, not as it was earlier as a functioning (and tamei) item.
Why isn't this similar to the cases of the mikveh whose water evaporated and the wine for terumah that turned sour. Both of the latter two are processes that developed over time. Shouldn't the rusted needle have the same quality, that is, shouldn't we be concerned that when it initially came to rest where it is found it was not yet rusted and therefore transmitted its tumeh?
In the needle example, the needle was found on top of Taharos (Rashi). We are in doubt as to whether the needle made them Tamei before it rusted. There is only "Chada l'Re'usa," that is, the only reason to make them Tamei is that the needle may have rusted very recently.
In the case of the Mikvah or the wine of Terumah, there are Tarti l'Re'usa. The person immersing in the Mikvah was Tamei/ the wine was Tevel, and besides that the Mikvah is now lacking / the wine is now spoiled.