Turns out I could delete the page and paste back in everything I managed to copy out of the page that I was editing. But what happened?
Just got a similar error on another page, but I don't know how I'm going to fix this one because I don't have the contents available to me to copy and paste.
By just sort of clicking around and editing the contents and canceling and saving I was able to get it to show me the contents of the card so I could copy/delete/paste and restore it.
Sorry, I was sure I had responded to this.
I have seen this before but have never found a way to reproduce it reliably, so I haven't been able to figure out how it happens.
I do know that the outcome is that the "current_revision_id" in the cards table ends up with an id that isn't an actual revision. We're re-writing the revision handling now (to track name and type changes), and that field won't even be there any more, so I expect this bug will get squashed in the process.