Ecplise - Population Cubes full on return
This is from the official FAQ on BGG: (italics added by me)

Q: What happens if I cannot return a Population cube to a track because it is full? This might occur if you return cubes from grey squares to different tracks than where they originally came from.

A: The track may be filled completely, so that no number is visible. This means that you do not have any production of that Resource. If a track is full and you still need to return cubes to it, the cubes must go to any other track(s).