It’s riddiculous having to create a trhead just for this but…
I think there should be a headcrab jumping at the player at the 7 minutes and 40 seconds of this video : (https://www.youtube.com/watch?v=A7ZPDoU1X_U&feature=relmfu).
The headcrab would be sitting there in the elevator waiting for Gordon to appear.
[I’ll take this trhead as an opportunity to say that you guys should insert more ‘scary headcrab that i did not see coming out of nowhere, omg I almost sh*t my pants’ moments. Because those are awesome. (And you’ve missed a lot of opportunities to insert moments like these into your game) ]