If someone wasn't an expert and was rebuilding or restoring an old Harley, like a Knucklehead, are there any parts or pieces that might be easy to mistakenly put on backward, upside down, or wrong without realizing?

It doesn't really matter what part of the bike it is, or whether or not it would affect how the bike runs (or doesn't) as long as an expert, looking at it would be able to tell right away.