When trying to install 499.1204, if BYOND is running the popup to close it comes up. IF the 'really close BYOND?' dialog shows up, and then BYOND is closed (in this case, I had the pager running), installation will fail if you click 'yes' to close the process, as it has already terminated.

My suggestion to fix this is add one last 'is BYOND Running?' check, before the kill-process call. That way if BYOND closes at the last minute, the installer doesn't just glitch and say it couldn't close BYOND just because BYOND had already closed.