I just finished assembiling my 8x20 lifetime shed.  I didn't run into any issue until the end when I wasn't happy with the gaps around the top of the doors under the gable.  I ended up removing a few screws and using a heavy hammer to better seat the gable and reduce the gap to a good margin.  If yours doesn't seem to be fitting during assembly it is worth wrestling and or pounding it into place before proceeding.  Overall the shed is great and I am very happy with the design and construction.