The #fediverse is a fascinating place. I was using #nostr and came across a post by @cstross but I couldn't reply to it on Nostr and I couldn't find it on #mastodon
I suppose technically this isn't a fediverse issue as it isn't really related to the way #activitypub works, but rather more related to the way bridges work.
Now I'm guessing this occurred because the post in question was deleted by Charlie (though I saw nothing untoward about it) or because it was a very good imposter on Nostr.
If it's the latter , it doesn't explain why the post seemed to come from Mastodon, albeit not from Charlie's usually address.
If it's the former then that is more understandable but maybe worrying, because it means deletions on Mastodon are not respected elsewhere.
Now Charlie being charlie, there's no way he isn't already aware of this, but I think it might be worth spreading the word to others.