Back when I was writing about ablaut reduplication, I read this article on the BBC website, which mentions this unwritten rule, and other structures that we use unconsciously. It’s interesting and worth reading, but I was struck by one passage in particular:
There are so many tenses you can use without even thinking about it, and almost certainly without being able to name them. It depends how you count them, but there are about 20 that you deploy faultlessly. The pluperfect progressive passive for an extended state of action that happened to you prior to another action in the past is, when you put it like that, rather daunting. But then you’d happily say “I realised I’d been being watched” without breaking sweat or blinking.
Eh, not quite without breaking a sweat, I think! Certainly the sentence is grammatically correct, and an example of the passive voice. The active version would be I realised someone had been watching me. We then change that to passive by changing the verb to the past-participle form (watched) and using the appropriate form of to be (had been being – past perfect continuous). But is it really that easy for even a native speaker to say this, and how often would we really use this form?