It looks like you have two faces on top of each other. Try selecting the face (the dark-ish one) and deleting it (x > faces). Is there still a face there? If so, then most likely you extruded, then left clicked or "undo"ed, but an artifact of the extrude was left over. From my limited experience, when you extrude, Blender seems to do two steps... First it creates the new verts and edges, then second it "pulls" them up. If you then "undo" the extrude, it sometimes only "undoes" the pull, not the creations of the new verts and such. If you then extrude again, it may pull up two copies of everything, or something like that. I am just speaking from my own experience, not from any inside knowledge of how Blender works, so good luck.