I think it was associated with deleting posts, since I deleted three test posts and there were three instances of the error.
That would make it absolutely unusable for my site. I’m not trying to use Discourse as a harder way to write posts to the fediverse than just writing them in Mastodon. Trying to force my entire community to start using [note]...[/note] bbcode annotations, including the frequent new site users seeking help, would be a non-starter.
No, I definitely can’t. It’s not about me. It’s about my site users, who are Discourse users, not discourse/discourse-activity-pub.git users.
It’s just an edit.
Federating edits is normal. To the best of my understanding, most of the widely-used ActivityPub implementations out there have allowed edits for a long time. Mastodon was the odd hold-out among popular implementations until last year when they finally caved and allowed edits.
I recognize that edits weren’t part of the MVP release, but until it either federates edits or gives me the option to turn off making the note uneditable, I can’t turn this on for my site. I don’t know whether my use case is weird or normal, of course.
My own goal for using the ActivityPub plugin is to make it easier to follow and share content as it is being created by my community forum members using Discourse. It’s not to use Discourse as a weird uneditable post-only replacement for Mastodon. 
My understanding is that you do intend to eventually support federating edits, since the error message says “not currently supported” — is that right? In that case, this is all just a temporary limitation and our difference in use case are not important. But it does make me wonder about investing effort into protecting edits vs. supporting federating edits.
I had forgotten to remove the retort plugin from my sandbox site when I removed it from my production site. Removing retort fixed both the weird error log and the weird error message. My mistake!
Does this mean that as the plugin stands today, we could use the same actor for multiple categories? Or that the data model is very flexible and the implementation details are easy to change?
FWIW in my use case, my preference would be to post the whole thing without any limitations. I know that’s not how others might configure it, but I think in the end I would set the max size to the same as the max size for a post on my Discourse, which is… large!