I ran across this expression recently in some Popeye cartoon [EDIT: it was Never Sock a Baby, 1939], but it seems to have been a recognized slang expression with maybe even a bit of old-timey flavor before then.
"Enoch," said Mr. Stratford, with a smile, "you said too much that time."
"Perhaps I did," said the old farmer, "but slips don't count." —The Hundredth Man (Frank Stockton, 1886)
Watch my smoke when I get into these Hood Shoes. They don't cramp a fellow's style when he's in action. Let the lads on the other team cry "Slips don't count!" Slips don't count against our score for our fellows all wear Hood Shoes. —Hood Shoes advertisement, 1923
What does the phrase mean, and where/when did it arise?