Right after you get the gluon gun in the Lambda complex, you go down a long elevator, and when you get off, you’re immediately attacked by houndeyes and the elevator doors close behind you.
BUT…
If you walk out, and then immediately duck back into the elevator to let the houndeyes come to you, the doors close and trap you in the elevator. The glass won’t break and the elevator’s buttons won’t operate. You have to restart from a save point. May not be a bug per se, but I thought someone should know that can happen.