May be caused by incorrect szs compression, the level not existing, or the game calling on objects it shouldn't (an Octo Expansion map calling on a Versus object).
May be an issue with rail connections (though rarely the rail itself). Make sure the parameters of the object you are attaching to a rail are equivalent to those found in an official level.
May be caused by Map.pack not containing a folder called Map with the relevent level inside of it. It must be found within that directory.
Make sure the team parameter is set a intended. You can do some interesting things with Ink Pistons that don't harm, or boxes that you cannot break, but make sure you are doing them on purpose.