If you take too long time in a room...

[Hunter of Shadows]

In my first play trough of Amnesia, i took quite a while in the sewers in the room called Cistern. And when i got the puzzle down, the room got put into the "shadows" mess and the water had water monsters in it.

In my second playtrough i plowed threw the room, but then no shadow.

What other rooms have this?

It's dependent on how low your sanity is, but other then that it's a random effect