I thought perhaps it was a special monster - or a Shadow chase scene... certainly tense nonetheless. Also thought I was being attacked after landing (slash of red on screen), before realizing it was just the fall.
Every time Frictional introduces something new, I'm terrified. I knew the normal monster AI wasn't smart enough to be in a duct with me, but I was honestly scared the first time that they had coded something new (like the spiders in Penumbra) that would follow me in the ducts. Goes to show what masters they are at building tension and leaving the imagination up to the creation of horror.