The last edge doesn't extrude along the faces.
Sorry, for some reason my screenshot was not attached earlier,
As marked with the blue arrow in the screenshot, the last edge didn't extrude along with the faces. I face this issue many times.
That face is created because you don't have clipping activated in your mirror modifier. Without that option Blender basically treats the mirrored part as a separate mesh island, like when you have disconnected parts in the same mesh. With clipping it gets treated as one connected mesh and you won't get that face.
So undo the extrusion and click on the clipping check mark in the mirror modifier and try again. :)
Oo, that was because clipping was unchecked. Thanks for your help :)