In The Markenmore Mystery (1922) by J. S. Fletcher, a man was talking to a detective in a bar:
“Don’t forget, Mr. Blick—though a gentleman of your ability and experience needs no reminding of it, I’m sure—don’t forget that it’s always the unexpected that happens! The unexpected, sir!—Ah, there’s a great deal in the unexpected! No one knows, sir, what the morrow may not bring forth!”
I usually find this statement in a positive form, i.e "No one know what the morrow may bring forth", but what it's meant by making it in a negative form this time?"