A mark of the course is also an obstruction but will still hit them occasionally and then do a penalty. If the line is closed downwind but due to a tactical error you are forced to go through it then I would think you take the appropriate penalty!
Mark,
Read 18.2a (which goes something like);
When boats are overlapped at 2 boat lengths, the outside boat is required to give room to the inside boat to round or pass the mark
or obstruction.
Thus, the boat to leeward has to give the boat to windward room.
