I believe I may have seen a couple of mainline flights like that, in my decade with BA - either the flight wasn't "created" in check in mode (departure control system), or it somehow got deleted after creation. Very few people in BA would have the access to create/remove flights, and I believe it's mostly done automatically based on the loaded schedule. I would say it's rare, perhaps bealine has better info.
It might happen more often than we know, but it's fixed before it affects anything or anyone.