On a Cray system, when a user submits an interactive job there is a time window during job startup after creation of the ALPS reservation but before STDIN and the job shell session are connected (i.e. before the prompt returns) when the user can issue a Ctrl+C to the job and orphan the ALPS reservation. At that point, when the job is killed, the ALPS reservation will linger and have to be manually deleted.
Until then, PBS and ALPS will have an inconsistent view of what resources are available for scheduling, and many jobs will fail to start and get put into hold states.