I'm experiencing this problem when trying to export to 3DS. I've done my best to build a scene by isolating objects and copying them, then choosing to paste faces, as pasting the objects was not available. The scene looks good now but I can't export it, as this happens and exporting to DAE results in a mashed up mesh.
I've spent hours on this, and it was painstaking (I really look forward to a time when I can make a box selection and just delete masses of objects, vertices or faces from scenes!), I really hope I can export it and don't have to go through that lot again!
Can someone advise, please?