Corrupt faces when extruding cylinder 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.
it seems you have overlaping faces on the left size. so maybe, you have extruded 2 rings together. 
You have 220 vertices but only 9 rings... that doesn't match. And also you have 40 vertices selected.