Why do your darks and reds come out with what looks like oil stains on them after you launder them?

You may have had a tube of vaseline or blistex or lip baum or some other oil based product in a pocket in the laundery that got on your clothes but if you so much as drop a chip on your pants it can leave an oily stain which just looks like a darker area. Darks and reds don't just come out of the laundry with oil like stains for no reason. Did you check the inside of the dryer? Could the dryer be on the blink and leaking some kind of grease agent?