A minor issue: we can't easily unassign people if they remove a "fixes #123" from the pull request body or from a commit message (force push). Unless there's a way to notice this in the API that I'm not aware. Otherwise we've have to keep track of it somehow. It's not impossible (we could put it on the issue comment), but it would be more work. Presumably people can unassign themselves, though, so if they remove one they should be able to fix it.

We can easily unassign people if the PR is closed without merging.

A somewhat less minor issue: if someone unassigns themselves for whatever reason, the bot should be able to respect that and not reassign them on the next event. I'll need to see if this information is accessible in the API.

GitHub has just the API: https://developer.github.com/v3/issues/timeline/