Another interesting one, with one common usage and one not-so-common, but still the ability to be confused. And they actually have fairly opposite meanings, so you don’t want to be going around confusing them!
To prescribe is, of course, firstly to issue a medical prescription – “the doctor prescribed me a new sleeping pill to reduce my nightmares about grammar mistakes”, but the definition that causes confusion is the one that means “recommend with authority”. I think it’s a bit stronger than that, actually. For example, in my English Language studies, we often talked about reference books being descriptive (describing what language was doing, including changes, including those that people might not like – new uses or coinings, the dreaded text-speak, etc.*) or prescriptive (describing what language SHOULD be doing and laying down the law about it (like I do in these posts … but more so). So that implies a slightly stronger meaning than simply recommending with authority.
Having gone on about prescribing, the other one is fairly simple. To proscribe is to condemn or forbid. So your Mum might proscribe the wearing of mini-skirts, or a company might proscribe inter-colleague relationships.
One letter, opposite meanings!
* of course, the dreaded text-speak didn’t actually exist yet back in the Dark Ages when I was a student. But you get the idea. It was probably L33T Speak** that people were getting upset about)
** ask a techie who’s around 40.
You can find more troublesome pairs here.