--disable_abort_on_error

--disable_warnings
DROP TABLE IF EXISTS t1;
DROP TABLE IF EXISTS t2;
--enable_warnings
CREATE TABLE t1 (f1 BIGINT, f2 BIGINT, f3 BIGINT);
CREATE TABLE t2 (f1 BIGINT);
INSERT INTO t1 VALUES(0,0,0);
INSERT INTO t2 VALUES(5),(6),(7);

delimiter |;
eval CREATE TRIGGER trg BEFORE INSERT ON t1 FOR EACH ROW
BEGIN
   SET @counter = 1;
   SET @my_max1 = 0, @my_max2 = 0;
   # The problem disappears, when the next line is deleted.
   SELECT MAX(f1), MAX(f2) INTO @my_max1,@my_max2 FROM t1;
   SET new.f1 = @my_max1 + @counter,
   new.f2 = @my_max2 + @counter;
   # We generated a replacement value, so @my_max<x> is already used.
   # Increment the number to be added next time.
   SET @counter = @counter + 1;
END|
delimiter ;|
SELECT * FROM t1;

# harmless - INSERT SELECT one record
INSERT INTO t1 (f1,f2,f3)
SELECT f1,f1,f1 FROM t2 WHERE f1 = 5;
SELECT * FROM t1;
#
# bad      - INSERT SELECT two records
INSERT INTO t1 (f1,f2,f3)
SELECT f1,f1,f1 FROM t2 WHERE f1 > 5;
SELECT * FROM t1;
#
# harmless - INSERT two records
INSERT INTO t1 (f1,f2,f3) VALUES(8,8,8),(9,9,9); 
SELECT * FROM t1;

DROP TRIGGER trg;