Saw an interesting misunderstanding the other day. Someone approached a user asking them to add alt text to their images. However, the images *did* have alt text, in a format ActivityPub supports, but that Mastodon doesn't know how to read.
I suspect people will encounter more and more of this as there are more ActivityPub-based programs that are doing things beyond just what Mastodon represents, and we'll have to get used to knowing that our view of a post might not be how it was authored.