Arch Linux, x86_64, scanbd 1.3.1, sane-git cca July 2013.
when I press an action button on the scanner before the action script (child process to scanbd.service systemd unit triggered by the previous press of a button) finishes, it is ignored. This might be expected behaviour, however, scanbd continues to ignore any other presses since then and the service unit has to be restarted to make it work again. Turning scanner off/on sometimes help, other times not.
Sending SIGUSR1 and SIGUSR2 stops and restarts the polling, however it does not help to solve this bug. Note that the debug messages in idle mode (no buttons pressed) are the same so it seems that the daemon continues to poll the scanner - it just does not "see" the presses at all.