Just another hairball photo. Many players are surprised to learn that a fiddle can have a hairball rolling around inside it, yet it is very common. This one, just above the loose soundpost, is actually fixed in place by rosin that has fallen in through the f-hole.
Some luthiers worry about these hairballs. I know folks who replace them after doing the repair work and others who collect them. I usually just take them out and throw them away, thinking they are something like 'dust bunnies' under the bed. So far, I haven't had any customers who expressed any interest in keeping them, either in their instruments or out.