old Spanish customs

Unorthodox, unregulated, or unauthorized practices that are nevertheless widely accepted and long-standing. Because this department existed long before all the new regulations came into effect, there are quite a few old Spanish customs held onto by the more tenured staff that, while not in the service of efficiency, aren't likely to disappear anytime soon.

old Spanish customs (or Spanish practices)

long-standing though unauthorized or irregular work practices.

This expression has been in use in printing circles since the 1960s; it is often used humorously to refer to practices in the British newspaper printing houses in Fleet Street, London, formerly notorious for their inefficiency. The reason for describing such practices as ‘Spanish’ is not known.

1998 Spectator [Outsourcing] can do much for flexibility and more for costs and it is a proven cure for quaint old Spanish customs.