Ear piece metal connectors: You can see them on step 22. If those are not bent up enough (laying flat) you won't have contact with the logic board... and no more audio from the ear piece. This is what happened to me. I had to unmount it again to bend them and hop the ear piece worked again!
3G and 3GS camera enclosure: As I was transplanting a 3G cover onto a 3GS, I realized as I was trying to re-assemble the camera that it would not fit: the 3GS camera does not fit in the 3G enclosure. Luckily, you can remove just the camera enclosure from the 3GS and transplant it to the 3G. It's dirty, I broke the ring, I transplanted some of the double face tape... but in the end it worked.