"Kelly E Jones" wrote
discovered that the relay had stuck 'on' (the contacts
were in the 'on' position despite no juice to the coil). The relay is
being used within the current/voltage for which it is rated. A gentle
push with a screwdriver separated the contacts, and after that they
seemed to work fine.

Any idea what caused this, and how to prevent it in the future?

Any kind of ring/arc supression circuit on those contacts ? I did once come
across a case where the contacts welded lightly shut when the supression
failed.