Bubbles. Never put bubbles in a cache. I've seen a few of those little plastic champagne bottles full of bubble stuff that you get at weddings. They make quite an interesting mess of cache contents after they freeze, crack the bottle, and thaw.
I guess it seems like a nice enough idea in the summer, but you just can't rely on them leaving the cache before the frost hits.
Using pencils in a cache instead of pens is an excellent idea, and don't forget to put a small pencil sharpener in, too.
- hamgran