It grew mould, or it smells
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Mould on driftwood
Wood holds water for a long time and supports growth while it does. Sealed before it is dry, a piece of driftwood becomes a mould culture in a clear jar.
Smell from a shell
Usually organic material left in the whorls. Rinsing the outside does not reach it, and the smell appears weeks after the shell seemed clean.
Clumped sand
Sealed damp, it sets. For a sand collection whose whole point is the texture and the colour, that is the sample ruined rather than degraded.
Salt bloom
A white crust appearing on a shell over months, which is salt that was never rinsed out migrating to the surface as humidity cycles. It also tells you the jar is damper than it should be.
And anything metal in the same jar corrodes
A shark tooth is fine and a dug bottle cap is not. Salt plus humidity plus a closed container is a corrosion cell, which is why sorting by material before sealing matters here as much as it does for coins.
The fix is always the same
Open it, clean it properly, dry it for longer than feels necessary, and reseal with a desiccant. The container is rarely the cause.
And the prevention is one habit
Never seal a find on the day you found it. Clean and dry in an open container first, and let the jar be where it ends up rather than where it travels.
Freezing is not a cleaning method
It is sometimes suggested for pest control and it does nothing about moisture, salt or organic residue. The find still has to be cleaned and dried afterwards.
Check the collection once a year
Open a sample of jars, look and smell. Every failure here is slow and reversible early, and nobody finds it without looking.