In the massive encounter right before Xen, a scientist walks out onto a catwalk to explain that he must not be interrupted and Gordon shouldn’t enter the beam early. He then walks over to a glass-walled control pod to work the generator right before flying aliens spawn in.
That scientist can be killed (I tranquillised him) but the game doesn’t end like it would in the original Half-Life. There, you’d get “Subject failed to utilise mission-critical assets” or some such over a black screen, and they you’d be bounced back to your most recent save. Here, the game is simply stuck in an unwinnable state. And I can actually save in this state.
I think killing the scientist before he gets to his controls should end the game.