So, I created a cylinder and extruded one of its ends a couple of times in one direction (right in image 1). Works fine. But when I extrude the other end in the other direction, the faces become "corrupt" (see the part to the left in image 1). I see no logic in this, and since I am following a very basic tutorial, I understand that this is not expected behaviour. Note in image 2 that the problem part is kind of darker in wireframe shading.
Can someone give me a clue? Many thanks.

