This has happened to me now on 3 separate games, so I can only imagine it's a bug. I've had the game frame setting turned off in the scene editor and whilst trying to play thorough a couple of scenes, I get stuck and can't see anything in the way. It turns out it's the game frame setting that has automatically changed itself from 'pass' to 'block'. Reading through the BB2 Bug forums, it looks like this is one of many game frame bugs.
We just figured the cause of this... even when hidden the game frame is editable. Will be fixed in the next release.