Speaking of the Brain Tumbler level...
An interesting thing about the Brain Tumbler Experiment level is that the Coach's level/doorway is closed to you in the Collective Unconscious, even though you've already been in there during Basic Braining. Since it's unfinished (you didn't have the Cobweb Duster the first time through), you would think the doorway would remain open in the Collective Unconscious.
And then, after you've been through Sasha's Shooting Gallery Level to gain the Psi-Blast power, Sasha's level/doorway is open to you when you go back to the Collective Unconscious to go back into the Brain Tumbler experiment to shoot the glowing eyed monster. But, again, the Coach's level/doorway is closed to you...even though Sasha's is now open.
You can't re-enter the Coach's level until after you've been in the Brain Tumbler for the third time, with Levitation, and, I believe, have completed the first tank battle.