I never am able to finish one, because I always forget up the proportions on the exterior.
That happens with me too.
I'll also tend to make one silly mistake somewhere, and everything will be out of place, usually by just a bit.
It annoys me loads, but I still can't be bothered to go back and change it, and just sit their hating my Build.
Interiors are the things that really stump me though.
I can build the outside of a Building fine, but the moment I have to decorate the inside with whatever (Whether it's Chairs, Tables, TV's, Sides or anything) I just get stuck on how to get everything to be a good proportion and still look fine.
That 1x1 plate may fit for a Computer mouse, but it's still too big. :/