Admiral Piett
Piett captain of the Executor is promoted straightly to Admiral in ESB (despite jumping a few ranks like the Vice Admiral rank). His rank insignia use to have 3 red bars on 3 blue bars, it now has 6 red bars on 6 blue bars. In ROTJ, when we briefly see him ordering his star destroyers to hold, he only has 3 red bars on 3 ble bars.
It is not said on any source that he has been demoted in ROTJ, but that he has kept his admiral rank. Whats is all this about?
Looking at a site for rank isignias, I saw that he was captain, then got promoted to Fleet Admiral (commander of death squadron fleet) and then demoted back to captain.