I'm still to new to it all. All the "mistakes" I make are more just trial and error (which is probably what alot of people would say) but I don't look at anything as a mistake.
However, I'm sure that the people out there that have been doing this for years and years have a few things they would go back and change if they could. I've heard (read) some people say "don't go into business with friends or family" "don't fuck up your adsense accounts with shady crap, unless you really know what your doing, otherwise you could be banned for life"...