Audio/video sync
You have lived a charmed life.
Synchronized audio came to film in the 1920s, and lasted (for TV) until the early 21st century. It was there that I first noticed significant lack of sync, but not via a Digital TV. It was apparent;y a case of the networks using DTV to distribute programming, but still transmitting in traditional (NTSC in my case) analog form. I knew something was up when my old CRT-containing TV "got the bug", a year of more before DTV transmission (other than tests) for over-the-air. Synchronized audio/video is pretty rare these days.
I'm still not sure how the arcane art of audio/video sync as lost. Did the last person who had read a spec die?
If any of them get thawed and re-animated, I'd also love to have a phone conversation devoid of long-delayed (sometimes 10 seconds or more) echoes and twilight-zone worthy audio artifacts.