I've seen at least three tickets over on GitHub where people are having issues installing Phabricator due to a rare? Mysql setting. Maybe it can be detected and warned upon.
Revisions and Commits
It's not very rare, it's just that MySQL (by default) will allow non-standard behavior for the GROUP BY clause.
The query from github issue 689 groups on the p.id column, but it's impossible (from the database's point of view) to tell which values to pick for all non-grouped columns. With ONLY_FULL_GROUP_BY turned off, you can basically get any value that it in a grouped set of rows.
Practically speaking, this probably isn't a big issue. However, it can lead to very strange (and difficult to find) bugs.