Dating back to the early 20^th century, the American-English phrase don’t spit: remember the Johnstown flood ¹ was jocularly used—or sometimes allegedly used—as cautionary advice in theatres, saloons, hotels, etc.

¹ On Friday 31^st May 1889, after several days of heavy rainfall, a dam on the Little Conemaugh River broke, flooding the town of Johnstown, Pennsylvania; this resulted in over 2,200 deaths.

This phrase originally occurred in association with don’t smoke: remember the Iroquois fire ².

² On Wednesday 30^th December 1903, a fire at the Iroquois Theatre, in Chicago, Illinois, resulted in over 600 deaths.

The earliest occurrence that I have found is from Hollidaysburg Happenings, in the Morning Tribune (Altoona, Pennsylvania) of Saturday 20^th June 1908—Hollidaysburg is a borough in and the county seat of Blair County in Pennsylvania:

Manager Louis Craine has hung two placards on the walls of the Pastime theatre down on Montgomery street. One of the placards reads: “Don’t smoke. Remember the Iroquois fire.” The other reads, “Don’t spit. Remember the Johnstown flood.”

This anecdote was popularised by the following from Everybody’s Magazine (New York: The Ridgway Company) of July 1908—many U.S. newspapers reprinted either this item or variants of it:

The enterprising manager of a little lyric theatre in northern Pennsylvania believes in profiting by the misfortunes of others. One day he displayed the following sign in his house:

DO NOT SMOKE
REMEMBER THE IROQUOIS FIRE

So great was the efficacy of this that before the end of the week he put up another:

DO NOT SPIT
REMEMBER THE JOHNSTOWN FLOOD

The phrase also occurs in an article about The Hereafter, a stage show produced at Dreamland, an amusement park at Coney Island, Brooklyn—article published in The Brooklyn Daily Eagle (Brooklyn, New York) of Tuesday 7^th July 1908:

A midget passes up and down the aisles warning the men not to smoke.
“Remember the Iroquois fire!” he exclaims. Then if a wag happens to be in the place, as there was last night, you’ll hear also: “Don’t spit! Remember the Johnstown flood!”

One of the numerous variants of the anecdote first recorded in the Morning Tribune (Altoona, Pennsylvania) of Saturday 20^th June 1908 occurs in the column Heard About Town, in The LaHarpe Journal (LaHarpe, Kansas) of Thursday 16^th July 1908—the following is about a local photographer, I. L. Harvin:

Recently he found his waiting room decorated with signs, of which the following are samples. “Do not smoke. Remember the Iroquois fire.” “Do not spit. Remember the Johnstown flood.” “In the dark room. Whistle. (Whistle furnished for ladies.)”

The following variant of the original anecdote is from The Evening Record (Hackensack, New Jersey) of Monday 3^rd May 1909:

The Winter’s Night Club held the closing dinner of their season at the Hackensack Golf Club on Saturday night […].
[…]
Mr. Mallon’s story of the boy who after seeing the sign in a Chicago theatre “Don’t smoke; remember the Iroquois fire,” and then wrote under it “Don’t spit; remember the Johnstown flood,” was probably one of the best bits of humor of the evening.

Yet another variant occurs in the column The Stroller, in the Akron Beacon Journal (Akron, Ohio) of Monday 20^th September 1909:

Two great catastrophes which resulted in great devastation of life and property a few years back were brought to mind the other day by a sign in one of the hostelries of the city. In one of the retiring rooms of this hotel a sign blazed forth the reminder, “Do Not Light Cigars or Cigarets Here; Remember the Iroquois Fire.” Underneath, some ingenious visitor, evidently possessing a sense of humor as well as a talent for wielding the crayon pencil, impressed the following: “Do Not Spit Here: Remember the Johnstown Flood.”

Occasionally, the phrase associated with don’t spit: remember the Johnstown flood was don’t smoke: remember the Chicago fire ³. For example, published in the Santa Rosa Republican (Santa Rosa, California) of Monday 6^th June 1910, a letter mentioned the following signs on the walls of a nickelodeon:

“Don’t smoke—remember the Chicago fire.” “Don’t spit—remember the Johnstown flood.”

³ The Great Chicago Fire began on 8^th October 1871, and burned until early 10^th October, devastating an expansive swath of the city of Chicago, Illinois, and killing about 300 persons.

The U.S. journalist and literary critic Henry Louis Mencken (1880-1956) allegedly contrasted don’t spit: remember the Johnstown flood with the British-English phrase these basins are for casual ablutions only. The first occurrence of this allegation that I have found is from the column Leaves from the Journal of a Quiet Man, in the Marengo Republican-News (Marengo, Illinois) of Thursday 24^th January 1957:

Instead of brusque Keep Off The Grass, the British are likely to say, The Public Are Requested To Keep To The Footways.
H. L. Mencken contrasted a sign in the washroom of the British Museum that said These Basins Are For Casual Ablutions Only, with one in a small U.S. railroad station that said Don’t Spit, Remember The Johnstown Flood.

However, H. L. Mencken did not actually contrast those phrases in The American Language. A Preliminary Inquiry into the Development of English in the United States (New York: Alfred A. Knopf, 1919)—the following is from page 162:

There is more than mere humorous contrast between the famous placard in the wash-room of the British Museum: “These Basins Are For Casual Ablutions Only,” and the familiar sign at American railroad-crossings: “Stop! Look! Listen!”

And the following is from page 302:

The national [i.e., U.S.] talent for extravagant and pungent humor is well displayed in many of these maxims. It would be difficult to match, in any other folk-literature, such examples as “I’d rather have them say ‘There he goes’ than ‘Here he lies,’” or “Don’t spit: remember the Johnstown flood.”