After the first glass tube, the corridor is so dark, I almost missed the other end. I had to come look at the picture here to see where it was...

Damn, your right, even without the level editor open I can remember it well enough to know your spot on. That's on the to-do list for tonight!

I probably am not very perceptive, but if you enter the first glass tube without placing any portals and without the cube you will be trapped. Entering the second glass tube without the cube will also trap you. It mostly effects those who will traverse with curiosity first, forgetting the necessaey objects that were at their disposal, like me.

Id love to know where and how you got stuck.
I had already discarded several other designs for many parts of this level due to not being able to prevent people getting stuck.

I did leave a few spots dark for atmosphere, but that can be tweaked easily enough.
And you are right, I have been playing with the repulsion gel a lot lately. I have been tinkering with the conversion gel in a few scratch/tinker chambers i never saved, but haven't yet had anything I've designed with the speed gel strike me as worth building into a full test chamber.

It is easy to get stuck and forced to reset. It is a little dark in some areas. It seems that your maps have a repulsion gel theme.