Unmapped user mentions.

[karliss]

When migrating comments done by users without mapping the tool will leave @gitlabusername string in the comment text. But such username might already exist on github. Resulting in sending a notification to completely unrelated github user.

It would be somewhat safer if all @something strings which don't have a corresponding entry in user mapping were replaced by _at_something. Downside is that slightly ruin comments mentioning an email address or having a code block with @ symbol. But I think that would be less bad than annoying a bunch of users who have nothing to do with your repository. And in most cases for human reader could still understand the intended meaning.

[ViliusS]

I support this idea. For the moment you can achieve something similar by gathering all your GitLab usernames and mapping all of them to @ghost GitHub account.