Someone one time wrote a great filesharing application. Then one day I asked him about caching what other clients downloaded and offered. A few exchanges later he concluded that one could reconstruct what someone downloaded. The problem was clients that didn't run very often. They needed some time to catch up with deleting things no longer available. They tended to wait for a good while for the other client to come back online so that they could get a new list. In the mean time you could ask them what a user had to offer and they would provide the outdated channel info.

The next day he released an update that erased the application and all data.