Meaning and origin of the phrase "slips don't count"

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)

Another example:

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?

Solution 1:

Some Google Books results strongly imply that the phrase originates from the children's game of marbles.

[We] played a lot of marbles at Rusk Elementary in Palestine in the Thirties. The games I best remember are Cat Eye, Four Square, and Bullring — "Knucks down!" and "Slips don't count!" —Texas Toys and Games, chapter "Marbles"

And:

You all know the saying youngsters use in marble games — "Slips don't count!" In this case slips did count. —The Fraternal Monitor, 1952–1953

I suppose the original meaning would have been something like "Come on, my thumb slipped! I should be able to re-do that shot." That is, a version of the mulligan rule. Similar house rules about "slips" might be seen today in billiards or golf.

The two examples in the question seem to be from long after the phrase became a stock phrase, because they're both punning on slightly different meanings of the word "slip" (a slip of the foot, a slip of the tongue) and don't seem to have anything to do with do-overs; they're just punning on the form of the phrase.

As for year-of-origin, I have no idea.