That's a very good point - genuine horror with humour rippling through it works better. Having something that's primarily comedy and then sticking a moment of horror in with no build-up is far less likely to work.

American Werewolf in London and Scream would be good examples. Both start with something quite gruelling, so that the director can put in moments of humour in the knowledge that the audience knows they're not safe. Any laughter is going to be nervous and laughing with the film rather than at it.