Here's one of those "yeah-right" things, of which Judaism is abundantly blessed, loosely translated from the Alter Rebbe's Shulchan Aruch, 228:29.
First, some background: grinding is prohibited on Shabbos. The Sages were concerned that if people could take medicine on Shabbos, they might grind fresh remedies on Shabbos. So, they prohibited doing anything which heals on Shabbos, even if the cure does not include any grinding, such as taking non-ground medicines, massaging oneself, gargling vinegar, etc.
Says the Alter Rebbe:
It is permissible to put a dry sponge or dry pieces of cloth on a wound if they are new, because they do not heal; rather they ensure that the [injured person's] clothing do not scratch the wound. Old clothes are prohibited, because they heal. When does this apply? when the [old] clothes have never before been on a wound. But if they were already on a wound, they will no longer heal--even though they are old--and they may be placed on a wound on Shabbos.