Does someone have a solution to this now....

I have been looking for a answer to this problem also, the thing is I know my faces are co-planar because after it happens i can turn on hidden lines and delete the triangulations without losing my surface.

Triangulation does occur from the intersect faces command as mentioned above however in some situations if I try to create an opening in a surface by drawing on the face with a rectangle or lines (my usual preferred method) then it does not cut any hole and also creates more faces across the entire surface (usually 2 extra surfaces on top of the original) in the same plane.

My firm recently moved to 2013 from SU7 and it is definitely has more bugs and issues while being slower to run in general.

I know this is an old thread, but I have the same initial problem as original poster. It only started happening when our office went from SU7 to 2013. However I used to get around it by saving back and opening the file in SU7 then exporting my model. Unfortunately SU7 is now creating the same problem. I should move to a more stable software, but have been using it for years now and is so quick with decent results.

I would love to know what causes this to happen.