A question arrived at Arqade from SU, and it had a lot of comments. I tried to migrate them to chat, but it didn't work, throwing the error:
Can't move comments from a user without an account
When a question is migrated, if a comment user doesn't have an account on the target site, the comment arrives with no author. This then means the comment can't be migrated to chat, as unlike comments, chat messages need an owner.
While fixing the authorship of the comment might be a bit much, assigning ownership of the authorless comment's migrated chat message to the Stack Exchange user (with a bracketed original-author) would prevent this from causing problems.