Doctor Who fans are probably the most aware of this. It’s hard to say which is the more stunning fact: how much of early Doctor Who was lost or how much of the lost managed to be recovered later.
Before magnetic tape for video really became feasible (1970s), to make a record of television broadcasts, the BBC (and many other broadcasters) would film a quality TV monitor with a film camera. Black-and-white film recordings exist of a number of Doctor Who episodes originally broadcast in colour in the late 60s and early 70s.
Black-and-white analog televisions, like used in the BBC telecine recorder, receive the same signal as a colour TV. They receive a signal with colour information, encoded at very high frequency within the otherwise black-and-white compatible TV signal. On a black and white TV, that high frequency colour information, shows up as fine variations in texture with repeating patterns. Shift the phase of the colour carrier, and the colour changes for an appropriately equipped colour decoder, and the pattern shifts on a B&W TV. It's almost imperceptible really, which was the hack that allowed compatible colour TV. But it's there on the B&W display. And it's preserved on the film recording, too. That colour data can be recovered with appropriate software:
https://en.wikipedia.org/wiki/Colour_recovery#From_chroma_cr...
https://www.theguardian.com/technology/2008/dec/11/digital-v...
What’s really wild is how in some cases, what was able to be recovered was a segment that was censored before broadcast as the film that was broadcast was lost, but the segment that was cut managed to survive.