I was doing the Waking Nightmare quest. I'd reached the part where I'd just broken the barrier. I talked to Erandur and then tried to save before going on to the next step, but I was thrown out of the game; it just closed down. I'm using the PC version.
The only unusual thing I'd done during this session was Alt-Tab to get to the desktop without closing the game.
I tried several times to restart the game, but the same thing kept happening, even after I closed all my programs and restarted my computer.
Next I tried to reinstall Steam from the disk, but this did not improve things. Finally, I uninstalled everything and started from scratch. I'm currently awaiting the results.
Why am I reporting this? Maybe as a warning to others, maybe in the hope that someone can explain what went wrong. Also because I need something to do while everything is being downloaded.
One thing I've learned from this is to save and quit rather than try to toggle to something else.

This is a known bug caused by the 1.8 patch.

It is impossible, or at least, near impossible, to save after drinking the torpur, and before the quest is over.

Once you drink it, you better get the rest of the quest done in one go.

On PC you can use the TCL command to pass through the barrier and disable it, bypassing the 'library/laboratory/torpor' sequence, and the related crash problems.