A missing ON (or comma join) produces a Cartesian product — every row times every row.

The comma-join is ancient valid syntax for 'Cartesian product', and a dropped ON in explicit-JOIN style degrades to the same thing in some engines (or errors in stricter ones — the errorless path is the trap). Small tables make it look like duplication; big tables make it look like an outage. The tell in results: every combination present, counts equal to the product of the sides.

SELECT
  l.lane,
  t.thrower
FROM
  lanes l,
  throwers t ORDER BY
  l.lane,
  t.thrower;

SELECT
  l.lane,
  t.thrower
FROM
  lanes l
  JOIN throwers t ON t.lane = l.lane
ORDER BY
  l.lane;

The league sheet paired every thrower with every lane — a comma join with no ON produced all 6 combinations, not the 2 real assignments.

Explicit JOIN ... ON always; comma-joins banned by convention; when a Cartesian product is meant (a spine × entities scaffold), write CROSS JOIN so the intent is loud.