I’ve seen in some productions, occasionally they even make do without a slate/clapboard and literally have someone clap on camera to sync the sound and image.

When I was working in the industry, often we had to prepare already-produced material for dubbing into other languages; for that purpose we would have to find “sync points” in the material to synchronize the new sound to. It was easier to do so with cop shows that had plenty of single gunshots and doors/car doors slamming shut. It was much more of a pain with dramas like “The L Word” where that stuff was less likely to happen.