I just finished putting the engine together and it looks great! However I have a lot of unusual pieces leftover. I did have a small headache when building it so i may have missed a piece here or there. The set doesn't fall apart so I know it's not a structural issue but it still bothers me. Anyone who put this set together already and can help would be most appreciated.

4 white 2x1 stud slopes

2 blue 1x1 smooth slopes

2 grey 1x1 cheese slopes

2 white 1x1 cheese slopes

1 black 1x1 cheese slopes

3 black 1x1 smooth slopes

1 black 1x1 circle

2 brown 1x1 circle

1 black droid arm

There are usually some spare of the smaller parts in sets

The provisional Bricklink inventory shows most of your spares as spares (I've not compared in detail)

http://www.bricklink.com/catalogItemInv.asp?S=10219-1

There does seem to be more than I would expect, are some to allow for motorising the set?

There's usually always one spare 1x1 cheese, tile, and round plate as well as Technic pins (and some other things like the small lever, not in this set) per bag, so don't worry about those. But I'm not sure what "white 2x1 stud slopes" you're referring to, I don't see any in the inventory and don't think I had any in my 10219.

That being said, either I missed to place a 1x2 black door rail plate – or had one extra... Oh, and I swapped several Maersk-blue or other rare pieces for ordinary ones in those places that are hidden anyway. :)

That being said, either I missed to place a 1x2 black door rail plate – or had one extra...

I got one too, don't worry. If I missed it in the instructions, it will be interesting to see what happens when I build my second copy of the set later today. For now though, it is at the front center of the yard truck.

Yep, a spare black 1x2 door rail here too. Quite a few spares actually, since there were so many bags.

I did have an error in my set though - got an extra black slope 45 1x2 but was short a white 1x2 with groove. So my loco currently has a hole since I built it after the containers.