[danzk]

Many TV stations would reuse tapes leading to many shows getting lost as well. The BBC routinely deleted content until it changed its archiving policy in 1978. Until then thousands of hours of programming in all genres was deleted.

https://en.m.wikipedia.org/wiki/Doctor_Who_missing_episodes

[johnwalkr]

This one is interesting in that most or all audio was recovered, and some episodes are recreated by animators.