Not the parent poster but it's queries like these:

Select user.id, user.name, sum(tx.amount) from user inner join txn on (user.id = txn.uid) group by user.id

User.name is not in the group by list so it is picked arbitrarily from the rows in the group. It's harmless here but not in all cases.

TBH I like the ability to do this. Sometimes you truly don't care which value you get, you just want one. Sometimes you have application-level constraints which mean the one it picks is guaranteed to be correct, but you can avoid the cost of enforcing that (at insertion time and at lookup time). It's a handy tool when it's appropriate.

But hell yes, tools should warn you about this loudly, as it's not generally correct.