The key antagonist in Keep on the Shadowfell is a cultist of Orcus named Kalarel. The players haven't heard his name yet, but they will soon.
Kalarel has a spy in Winterhaven. The module contemplates that the revelation of this spy will be a major surprise, and hinges some of its plot on the idea that the PCs will trust this spy until they are suddenly betrayed.
The module must be expecting some pretty dumb PCs.
The spy is an elf woman named Ninaran. She has no ties to any other NPC. She is described as "a hunter", which is D&D code for "ranger", and clues the players in that she has an honest-to-god character class. Her personality is described as "stiff and bitter", she "drinks alone" and "is not interested in conversation". She is the only person in Winterhaven who knows anything about the possibility of a "death cult" in the area, and is surprisingly forthcoming on where its headquarters might be found.
She may as well be wearing a carefully-lettered badge reading, "Hi! Ask me about being an obvious spy!"
The challenge for the DM is not how best to present her traitorous duplicity; it is how to stop the PCs butchering her like a sow during their first meeting. Players are not idiots and they are particularly receptive to a one-note stereotype. Characters who are "stiff and bitter" and who "drink alone" are unlikely to become the players' bestest buddies.
 Keep on the Shadowfell presents several characters in Winterhaven that the players have good reason to get along with and like, including a kindly old wizard, a matronly innkeeper, and a ditzy elf who picks flowers for a living. Did it not occur to the developers that any of these would have made for a more surprising and powerful betrayal?
 Ninaran is so obviously a traitor that in internet write-ups and games I have personally witnessed she was suspected before the players knew there was anyone for a traitor to work for. Was this issue not detected during playtesting?