Sometimes a corn cob pipe or a briar will get a burn out. This is when the fire burns clean through to the outside. Some people then go and throw out the pipe.
I take the pipe and fill it with ash and then some water. Make it a slurry and cake the hole up. Suck some flame through it and repeat. It forms a clay plug. Seems to work just fine. Heard of this recipe before in alt.smokers.pipes and it was called pipe mud. Seems appropos. And now my corncob lives to smoke another day.