Its only once in awhile and its right off the bat at the very start.
All routes start with a RI of 0, and in the beginning you have a CI of 0 (or near it). That means that your first routes start with very low LFs. You've got to drop the prices fairly far in order to turn a profit right from Day One, and not much you can do is going to fill that plane up, particularly if there's any additional competition on the route with a higher RI/CI. As your RI/CI increases, your LF will increase, and you're able to get away with raising the prices too.
Remember, with a RI or 0, that means people don't know about you. It doesn't matter what
you charge if your problem is that people simply don't know you exist, so lowering the prices only helps to a certain extent. The best way to get that plane full is just to ride it out until that RI gets north of 80.