Have seen a few different projects not supporting MySQL 5.7 because ONLY_FULL_GROUP_BY being turned on by default is breaking things (https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html).

That can happen when you start by deliberately ignoring standards and then slowly start implementing them as you realize why they were part of the standard (SQL-92 in this case) in the first place?

Previous behaviour: "the values chosen [were] nondeterministic, which is probably not what you want"