What is a piece of lost media that just no longer exist?

The Doctor Who story 'Mission to the Unknown' from season 3 back in 1966. While it was telerecorded for overseas sales (tl;dr Doctor Who episodes were mastered onto videotape but they telerecorded them onto 16mm film for overseas broadcast because film is a universal medium whereas videotape isn't), it was never sold and never aired overseas, so once the master tape was wiped for reuse (as videotape was expensive and the tape was more important than what was on it) and the telerecording was junked, that was it. The episode is likely gone forever.

I have to say likely because there is the slimmest of slim chances someone could find a copy of the episode, since the story The Daleks' Master Plan from the same season (the story that Mission served as an introduction to) was also never sold for overseas broadcast and yet three of its twelve episodes were later found but it's highly likely that shit is lost forever (except for the audio of the episode, so we at least have something).