I was chatting to my mechanic mate and he was, erm, “talking” about the bottom bracket used on a < name redacted > bike. That particular BB has two variants, internal and external. For most of the production run they used one but for one small batch they used the other but didn’t think to tell anyone! If you looked on that company’s website for the model you’d assume it was all straightforward but it was anything but.
Away from BBs & headsets, the most complicated thing on a bike is indexing gears, even then it’s a case of doing it slowly and methodically. Most other things are relatively straightforward.
*< name redacted > is one of the major manufacturers!