So I spent hours compiling ffmpeg. Which gave me a .deb, which I installed. The executables went into /usr/local/bin. All good, new ffmpeg working fine.
But next day... Server not working (rtmp -> mpeg-dash streaming) and after checking my usual points of failure I happened to check upon ffmpeg in /usr/local/bin...
Gone! (And a bunch of python files I don't remember seeing, but that might just be me.)
So I dutifully re-installed the .deb. All working.
But after a reboot today. poof! Gone again.
What dumb thing am I missing?