Of U.S. origin, the informal phrase an accident, or a catastrophe, etc., waiting to happen designates:
– a situation that is likely to lead to disaster or misfortune, especially because of negligence or neglect;
– someone who behaves in a way that is likely to cause trouble.
—Cf. also accidents will happen in the best-regulated families.

These are the earliest occurrences of the phrase that I have found, in chronological order:

1-: From the column With…. Solomon The Exchange Editor, published in the Central Kansas Democrat (Lyons, Kansas) of Friday 6^th October 1905:

We heard a man tell another one the other day that he “Stood around looking just like an accident waiting to happen!” We nearly fainted.

2-: From Turning The New Year’s Leaf, published in The Wisconsin Agriculturist (Racine, Wisconsin) of Thursday 30^th December 1915:

After all the mistakes and disappointments and business battles that leave us tired and sour and sick, there comes a new day, so that we can begin over again and do better. If we start such a plan, the coming year and those following it will be happier new years.
We should make new resolves. We should be determined to do our best. If you sit around like an accident waiting to happen, you will never be called “fortunate.” You will always be classed with the fellows who “always have bad luck.” Is it luck? Or will you be to blame, yourself?

3-: From Rube’s Column, published in the Johnston County Capital-Democrat (Tishomingo, Oklahoma) of Thursday 1^st June 1916:

Rube’s political judgement may not be good but no amount of talk would induce us to register as an “Independent.” An independent is an accident waiting to happen.

4-: From the Tucson Citizen (Tucson, Arizona) of Tuesday 24^th April 1923:

ARIZONA LINEMAN AND OPERATOR AT K. C. WED IN PHONE ROMANCE

Miss Therese Cartain and Patrick Mellon, principals in “long distance” romance. Miss Cartain at her Kansas City switchboard and Mellon “on the line” in Arizona.

Patrick Mellon, California State gridiron star, in search of romance and adventure, decided that trouble shooting for the A. T. & T. ought to provide both.
And it was out in the wilds of southwestern Arizona that Mellon buckled on his climbers and took up his duties as a telephone lineman. He found plenty of adventure.
But romance was scarcer than threshing machines in coral fields. But Mellon kept “shinning up” poles and down again like an accident waiting to happen. And finally it did.
He plugged in on a line. And the softness of a feminine voice almost knocked him off the pole.
The pleasing voice was making a brave but futile effort to “get” Los Angeles. Mellon, scenting romance at last, “cut in” and asked permission to aid the “lady in distress.” He managed to wake up the dozing Los Angeles operator.
Then the “sweet voice” called back, thanked him and said she was Therese Cartan [sic], switchboard operator at Kansas City.
That started the whole affair. After that little chats between Mellon on his airy perch atop a pole in the Arizona desert and Miss Cartain became frequent occurrences.
It’s certain that Mellon spoke of other things beside deserts, snow-capped peaks and open plains.
And among other things Miss Cartain must have said “Yes.”
If she didn’t Mellon must have decided to take a chance anyway, for it wasn’t long before he was speeding to Kansas City.
And how [misprint for ‘now’] they’re Mr. and Mrs. Patrick Mellon, if you please, and whether you please or not.

5-: From the Morning Oregonian (Portland, Oregon) of Sunday 9^th December 1923—the readers had been invited to provide the following cartoon with a caption:

MRS. A. A. CAMPBELL, 1163 Haight avenue, will receive a check for $10 in an early mail, her title, “About to Be Fired With Enthusiasm,” having been adjudged the winner in title contest No. 57. […]
There were a number of titles that deserved honorable mention. The title contest judges are rarely unanimous in choice of a title, and perhaps in the following list many will find a caption that suits them better than the prize winner:
“Doomed by His Own Sentence”—C. A. Reichaan, 507 Marion avenue, city.
“An Accident Waiting to Happen”—Robert Stanley, 926 East Thirteenth street North, city.
“The Outgoing Male”—Mrs. A. B. Todd, Forest Grove.
“Sole Stirring Thoughts”—M. L. Tims, 81 East Buffalo street, city.
[&c.]