Charles Ellicott Commentary Exodus 16:20

"Notwithstanding they hearkened not unto Moses; but some of them left of it until the morning, and it bred worms, and became foul: and Moses was wroth with them." — Exodus 16:20 (ASV)

It bred worms.—On the Sabbath it bred no worms (Exodus 16:24), so that we must view the result spoken of as a punishment for disobedience, not as produced naturally. Neither of the natural mannas is subject to any very rapid decomposition.