I would guess, that that edge was created erroneously while using the cut faces tool. Perhaps the user wasn't familiar with the tool's usage and didn't realize that when they cut face on an object that it slices thought eh entire object from the angle its viewed from, ...
Were you trying to make an edge across a specific face, of the object from perspective view and later discovered that the edge was through the full object in question?.. The two triangles that are smaller, and the face that all the verts on the edge shown are co-planar suggest this to me.

I ask because its a good learning point for new users, Even if not what happened to your mesh, I still do it on occasion, Lets say i have used the cut faces tool for something, step away for a moment, and forget to deactivate the tool while going to select another component in my scene, blam i end up with an edge through my model that looks just like what you have there,

Agian though, the solution would be more forthcoming if one can view the object as a whole to see what you were trying to achieve.