On my first grow (about 8-9 mo ago):
Used MG, never again
Had a male I did not recognize until way too late. It was a Caramelicious male that pollenated WW. Both were from seed from AMS, the WW was feminized. This part turned out well however. The resulting seeds, of which I must have a couple hundred, are awesome. I have grown about 6 so far.
I did not know about the fan leaves yellowing and dying off as the plant dumps chemicals etc in bud phase. I nearly wrecked my plants trying to correct that, before I found out what was going on.
I'm sure there were other newbie mistakes I made, but those are the highlights. I learn every day though, and that's a fact.