I put a cromulent plist file in /Library/LaunchAgents; I load it for the current user using launchctl bootstrap gui/501 $plistfile. Great!
But if I then log in as a different user, without rebooting, it doesn't run. I can't do a bootstrap for a user who isn't there; I can't do a launchctl load for an agent. This seems like I'm missing something, but googling hasn't helped me a lot.
(On top of all that, I am pretty positive this used to work, but I may be thinking back to MacOS not macOS.)
Ha ha! Figured it out. Someone set the stderr and stdout in the plist to be a file in /tmp; the first time it's run, it gets created. With a mode of 0644. This was causing it to not launch.
(I figured this out by pre-creating the file and doing a chmod a+rw on it, then loading the agent via the previously-used launchctl commands. And then when I logged in as the second user, there was the agent, doing its agenty things.)
Ha ha!