Dunno if this bug has already been highlighted:
In the tutorial (level 11 or 12 I believe) you are tasked with upgrading a building. When you mouse-over the material requirements to upgrade a building, it shows how much stone, wood etc. is required.
However the mouse-over reveals that you require HARDWOOD. At such an early level you haven't progressed as far as obtaining Hardwood!!! And in-fact are able to upgrade a building using your supply of Pine Wood.
Whilst this may not be a major bug - it is certainly misleading, especially to a novice.