I'm a fan of having horror and comedy in the same story, but keeping the two separate. Horror is still horrifying, but characters have moments of comedy (sometimes dark comedy) amidst the horror.
Aliens is one example, and if you ignore the TV-budget effects the Buffy TV series is another. The new Doctor Who series has some creepy/horrifying elements, and even entire episodes: "Blink", "The Girl in the Fireplace", "The Empty Child/The Doctor Dances", "The Impossible Planet/The Satan Pit", and "Human Nature/The Family of Blood", among others, spring to mind.
The key, I think, is to keep the threat credible, even though characters are wisecracking ... and perhaps the situation is absurd. Angelus can say (darkly) humorous things but he's still a monster; statues, clockwork androids, and a cursed child crying for its "mummy" are absurd, but they can still kill you.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)