A lot of SQL code I've read, it seems like the developer assumes that the default sort order always holds. For example when building an HTML select list they would just SELECT id, name FROM table without issuing an ORDER BY clause.

From my own experience it seems like dbms alway orders data using FIFO if no ORDER BY clause is given and no index. However, the order is not guaranteed. But I have never seen a dbms reordering data if there no change to the table.

Have you ever experienced a dbms selecting data in a non deterministic order if there is no change to the table?

Is it best practice to always put an ORDER BY clause?