This one came to mind yesterday:
Someone let the cat out of a wet paper bag.
Gio came up with a bunch of variations on the theme of letting the cat out of the bag:
We'll bag that cat when we get to it.
A rolling bag gathers no cats.
You can lead a cat to a bag but you can't make him drink.
Don't put the bag before the cat.
That's a whole different bag of cats.
And another from me:
I just opened up a bag of cats.