Mebbe a penny seaed into the wound site? Hmmm...?
OK, enuff. Let's get back to biology.
As Mike also said, not every wound will turn into a cavity. True. This is a crucial point. It is up to the individual climbing practitioner to determine which wound is ien route to becoming a cavity. I see this as a cavity by cavity assesment.
Which wounds might become cavities? Wounds whose bare-wood cross section begins to crack. Wounds whose bare-wood cross section <i>has</i> cracked. Wounds whose bare-wood cross section has visual signs of progressive decay. Wounds whose bare-wood cross section faces upward, or toward prevailing precipitation.
Here's a few examples: