Haven't made any too serious mistakes, I think... But I do regret selling me and my sister's childhood G2s for so cheap, they were all complete with accessories and in a nice condition. I didn't like G2s so I decided to get rid of them, and I thought no-one else would be excited about them either so I didn't ask much for them.
Well, at least they ended up finding a loving home. (And before you ask, yes I did have my sister's permission for selling her ponies, and I gave the money to her.
)
Most of my newbie mistakes are custom related, like not removing the original symbols so you could still see them under the white paint... lol. Well, I admit I could've added a couple more layers of paint, I was pretty impatient with my first custom(s).