Ok, hard to explain without diagrams, but here goes. Had a unit of marauder horsemen next to a forest, with a unit on the corner around the edge of the forest but not in the forest. So, I charge, and after shooting 3 are left.
Now, the problem is this. I charge with one horseman in base contact in order to avoid the dangerous terrain test. They argue that to obtain maximum frontage I have to go through the forest and take the test.