One of the biggest killers of indie projects, scope creep. As sticking to a release schedule was one of our pillars it can mean changes to gameplay.
For example, our original design included ladders, but when timeboxing out tasks, we saw how a simple design decision to ramps could help us with our schedule by reducing person hours like additional animations, camera scripting, extra QA, etc
After a new concept, and a quick implementation test we made the call. Ladders be gone.
Scope being a design pillar helps keep front and center what time you have left in your schedule.