I was going to do this big post breaking down the contemporary accounts of the winning play of the 1886 series and then contrast that with later accounts to show how the myth of the $15,000 Slide developed but Jon David Cash, in Before They Were Cardinals, has already done that and has done it better than I could have. So I'm just going to let him tell the story (and if you haven't read Cash's book, you really should; it's a good read and strong on the Four Time Championship era):
In a relatively recent publication, Lowell Reidenbaugh of the Sporting News described the winning play of the 1886 World Series: "The deciding run was scored in the tenth inning of the final game by Curt Welch. Newspaper accounts of the day reported that Welch scored on a wild pitch. [Modern] history has recorded the play as a steal of home."
The uncertainty surrounding the winning run apparently stems from an account given by Charles Comiskey. In a 1919 biography, Gustav Axelson included Comiskey's recollection of the decisive run. Comiskey stated that Welch was running on the pitch, drew a pitchout, and crossed home safely on a "$15,000 Slide" that gave St. Louis the entire gate receipts of the winner-take-all World Series. Later, Arlie Latham related a similar version of Welch's "steal" to author Robert Smith, who included it in his landmark 1947 work Baseball and other books he wrote on the history of the sport. Together, Axelson and Smith ensured that the former Browns' narrative of the "$15,000 Slide" passed into baseball folklore.
One contemporary newspaper report seems to corroborate part of the description offered by Comiskey and Latham. According to the Chicago News, as soon as third baseman Burns "gave Kelly a signal to catch Welch at third," the Chicago catcher responded by calling a pitchout. A mishap transpired, though, between the Chicago batterymates: "Kelly played away from the plate...but Clarkson put a ball over the plate which Kelly just touched with his fingers and bounded away to the grandstand, while Welch came in with the winning run."
Other nineteenth-century newspapers said nothing about a steal, but instead debated whether Welch scored on a wild pitch or a passed ball. While the Chicago Tribune considered the play a passed ball on the part of Kelly, three St. Louis publications-the Republican, Globe-Democrat, and Sporting News-claimed Clarkson had been guilty of a wild pitch. The Post-Dispatch tried to settle the question by simply asking Kelly. The Chicago catcher seemed willing to accept responsibility ("I would say it was a passed ball"), yet he also emphasized how difficult it was to catch this particular pitch: "I signaled Clarkson for a low ball on one side, and when it came it was high up on the other. It struck my hand as I tried to get it...Clarkson told me that it slipped from his hands."
One might suspect that Comiskey and Latham embellished their tales of the play when they later recalled it, some thirty-three and sixty-one years after Welch scored. But it would be a very smug act on the part of present-day historians to state definitively that they understood an event better than two eyewitnesses. Without photographic evidence of any sort, we must rely on the various first-hand accounts to recreate what may have happened on the "$15,000 Slide." Except for the reminiscences of Comiskey and Latham, no primary sources indicate that Welch was running with the pitch or ever slid across the plate. Still, Comiskey's detailed analysis of the play cannot be altogether dismissed as mere baseball mythology. In fact, the Comiskey/Latham explanation meshes well in some aspects with some of the contemporary newspaper reports. For example, the two old Browns agreed with the Chicago News' assertion that Kelly called for a pitchout. On Clarkson's first pitch to Bushong, Welch danced down the third-base line, straying so far from the bag that Kelly "could have nailed him easily" with a throw. Seeing this, Burns and Kelly cooked up a scheme to trap the reckless Browns' runner in the snare of a pitchout. Meanwhile, Comiskey, stationed at the third-base coaching box, encouraged Welch to take another long lead. This strategy distressed Clarkson, and it would have enabled Welch to get a great start for home on any grounder hit by Bushong.
As the scene actually unfolded, the first of these advantages came into play. Clarkson, aware of Welch's capability to steal home, certainly seemed to ponder the prospect. While Welch scampered down the third-base line, the Globe-Democrat observed the disturbing effect this exerted upon the Chicago pitcher: "Clarkson, who is usually so cool, was visibly nervous. He rolled and twisted the ball around in his hands several times before he got in position to pitch it." A distracted pitcher, under such circumstances, might balk in a run, uncork a wild pitch, or even miss his catcher's signal for a pitchout. It appears plausible that Clarkson simply missed the pitchout signal and later, rather than admitting his mental mistake to Kelly, presented instead the physical alibi of a pitch that "slipped from his hands." So at the same moment Kelly moved away from the plate in expectation of a low outside pitchout that would allow him to make a quick throw to third, Clarkson let go a high inside fastball designed to jam Bushong and force a futile infield pop-up. Kelly barely managed to reach back and get his fingers on the ball, but he could not prevent it from rolling all the way to the grandstand as Welch scored the winning run.
The only question left to resolve is precisely what Welch did once the pitch was released. It is possible to surmise that Welch instantly recognized the miscommunication between the Chicago batterymates, and as soon as the ball left Clarkson's hand, galloped ahead thinking he could reach home before Kelly ever got the errant pitch. However, Ed Sheridan, reporting in both the Republican and the Sporting News, strongly suggested otherwise with his brief notation: "Welch trotted home." This same comment also indicates that, in all probability, Welch did not slide home.