How about the Sun route? Solaris 2.7 vs. SunOS 5.7

quesera · on Jan 13, 2016

Even more fun..

  SunOS 4.1.4 was retroactively renamed Solaris 1.1.2.
  
  SunOS 5.6 was Solaris 2.6, but
  SunOS 5.7 was Solaris 7.

To Sun's eternal credit, they were smart about keeping the internal versioning and the marketing versioning separate.

Even to this day, so far, Oracle hasn't messed with "SunOS".