A musty item -

[jamawani]

What item did you find at the bottom of your pannier
that elicited an "Ewwwwww!" from everyone within 100 yards?

[jamawani]

Gotta be those wet socks from the week before.
Wrapped up in a plastic grocery bag.

[hikerjer]

Somehow some left over cheese got buried in the bottom of my back pannier for four days before I discovered it. Not  pleasant.

[John Nelson]

Not pleasant when your chain lube leaks all over your clothes.

[raybo]

I've learned not to keep half an onion in my panniers no matter what it is wrapped in.

[Pat Lamb]

I knew it was there, but I didn't realize how musty damp clothes could get in three days.  In my defense, there were sporadic showers the last two days -- impossible to get them dry.

[LouMelini]

Everything smelled musty from the heat and humidity of Kentucky and Virginia on the TransAmerica. I soaked the panniers in a mild bleach bleach solution when I came home. My panniers (ortlieb) smelled more musty than Julie's non-waterproof Lone Peak which brings up another discussion of pannier type in wet vs. dry areas of the country.

[El_Chupacabra]

I once put some damp swim trunks and Teva sandals in a plastic bag, so as not to get other stuff wet in the pannier.

That was a stinky mistake....  Better to just bungee it all on the outside and let it dry while riding.

[Ty0604]

Gotta be those wet socks from the week before.
Wrapped up in a plastic grocery bag.

+1 - Same...

I can almost smell them as I type this 

[BikeliciousBabe]

I use bandanas inside my pots to protect the surfaces from the stuff that nests inside (e.g., the pot gripper, spice vials). After my short trip back in July I neglected to unpack everything, forgetting that things had been a bit damp and that there was a damp sponge inside. When I finally did a couple of weeks ago, one of the bandanas was covered in mold. Fortunately, I was the only one who had to deal with it.

[JimmyTemp]

Gotta be those wet socks from the week before.
Wrapped up in a plastic grocery bag.

Damn, I've had the same situation, and that was disgusting scene...