A little too close for comfort.
I recently won some really nice lots at auction, and I needed to have them shipped to my APO address. For safety's sake, I had them shipped to family in the States, then placed in additional packaging for the APO trip. I never thought I needed to request waterproof packaging for an airmail trip.
I knew there was trouble when I picked up the package, and it was in a plastic baggie with the USPS "we're sorry, it arrive this way" label on it. The outer envelope (from family) and inner envelope (from the auction house) disintegrated due to water damage, and the internal cardboard stiffener was soaked through.
Luckily, the cardboard stiffener stopped most of the water, except around the edges. The plastic sleeves holding the stock cards were just enough to prevent the remaining water from damaging the stamps.

And just because, here's what was in the package.

Feldpost
Hela issue with plate flaw, Mi 13 b IV
Luftschiff Expedition, Si I corner

Kamerun
booklet, Mi MH3A