A function returned more than one row of values (for example, it executed RETURN...WITH RESUME) when the caller expected only one row of values.
Example of error:
CREATE FUNCTION testroutine (limit INT) RETURNING INT; DEFINE i INT; FOR i IN (1 TO limit) RETURN i WITH RESUME; END FOR END FUNCTION;
CREATE FUNCTION gettest() RETURNING INT; DEFINE var INT; LET var = testroutine (10); -- error RETURN var; END FUNCTION;