As hard as we try not to, it's easy for Game Designers to get overwhelmed by the schedule and first focus on getting a piece of the game fully implemented. People are always asking questions like "When are we getting that new encounter we talked about?" or "I thought we were changing the dialogue in that last conversation?" so a Designer feels a lot of pressure to get all of the components into the game first before taking the time to evaluate each piece and polish it into the experience we're shooting for.
So, at some point you reach the time in the project where all of the pieces are in and you can focus on polishing and getting in the details...This is when I tell a designer to "make it fun". This is just the point where some Designers tend to make a mental transition and can think more clearly about the player's experience. Now that this transition has been make, great difficulty comes when we find that a particular piece of gameplay just isn't working and needs some significant re-work. It is frustrating and mentally exhausting to have to go back and make these kinds of changes, not to mention the time generally isn't available here and causes extra hours.
While I do see this often with Designers, we should be working hard for this entire scenario to never happen. The more that a Designer can learn to think about the player experience from the very beginning, the less heartache there will be as the project is winding down.