You've probably all come across the expression "Don't buy a pig in a poke". You know that it's not wise to buy something sight unseen. Poke in this context means a bag, or sack. The phrase comes from medieval markets, where the local yokels could buy their livestock. If the market trader handed you your purchase of a suckling pig in a poke, it was a wise buyer who peeked in the poke to make sure that it did in fact contain the pig, and not some relatively worthless animal such as a cat.

Of course, if the fourteenth century apprentice huckster were clumsy or careless, he might drop the poke and let the cat out of the bag, giving rise to another expression, meaning to give away a secret...

Can anyone think of any more paired expressions like this?

The second part of this I'm not sure of (as I can't remember where I read it) but the first part is cited in The Concise Oxford Dictionary of Proverbs.

The phrase 'spoil the ship for a ha'porth of tar' was originally a dialect varient of 'spoil the sheep for a ha'porth of tar' and referred to the use of tar to protect sheep from sores and wounds becoming infected. 'Spoiling the sheep' here meant allowing the sheep to die rather than spend the money on the tar.

So far so good.

I've also read that the phrase 'tarred with the same brush' has a similar origin in that the application of tar to an already infected animal could then, if the brush were not cleaned in between result in the infection being spread rather than reduced.

I'm not sure how accurate that second derivation is though.

Habent Abdenda Omnes Praeter Me ac Simiam Meam