A list of cube locations so you can figure out which one you didn't pick up. Standing at each location I list you should be able to see a cube. It may be behind you, or perhaps hidden by a pillar so you can see only its pink glow.

1. Circle: big cubical robot sublevel, where you pick up the control stick
   
2. Circle: where you open the door to pick up the circle
   
3. Circle: next to the supercomputer, through a door you need help to open
   
4. Triangle: where you pick up the triangle
   
5. Square: where you pick up the square
   
6. Hexagon: behind the door you activate with two parallel laser beams
   
7. Hexagon: behind the door to the right of the door with the hexagon behind it
   
8. Hexagon: next to the octocube