The -- coments don't work properly if the comment is not followed by a query:
http://sqlfiddle.com/#!4/ee7da/3121
http://sqlfiddle.com/#!7/04eca/159
Or if the commented out line contains ";":
http://sqlfiddle.com/#!4/ee7da/3122
http://sqlfiddle.com/#!7/04eca/160
It seems to work fine for other databases but I only tested one version of each.