Flying in the face of tradition can be dangerous.
If you're thinking, "I'm not sure and I hate having to figure things like this out," LOOK FOR THE MAIN VERB (remember?). Now, where does the subject of the verb usually come? Right, immediately before it.
Can "tradition" be the subject? Not even you (if you hate the whole idea of punctuation and never do it right) would think that. So? Ask yourself, "what can be dangerous?" "Flying in the face of tradition," of course. "Flying in the face of the tradition" is the subject. (All right, wise guy, "Flying" is the subject, but it's a lot easier to think of the whole phrase as the subject. We're trying to make this easy, right?)