A couple of days into secondary fermentation the beer darkens up, white film develops on the top, and an almost rancid smell develops. This has happened a couple of times. Any ideas?
It could be an infection starting. Watch the gravity, a wild yeast or some bacterial infections will cause the gravity to go down pretty low as they eat up the leftover sugars. Let it go and see if the flavor and film worsen and that's probably your problem. Check your sanitation procedures and/or try to avoid secondary fermentation in the future and just leave it in the primary.