Although Jacob Marley, Ebenezer Scrooge's now-deceased former partner in the firm of Scrooge & Marley, gives Scrooge one specific warning in stave 1 of Charles Dickens's A Christmas Carol—“I am here tonight to warn you, that you have yet a chance and hope of escaping my fate"—Marley gives other warnings to Scrooge during his late-night visit.
Marley says that Scrooge's "chance and hope of escaping" Marley's fate is of Marley's own "procuring." In other words, Marley himself arranged for the visits of the "Three Spirits," which at first Scrooge politely declines. “I—I think I’d rather not,” says Scrooge.
This is followed by another warning from Marley that Scrooge "cannot hope to shun the path" that Marley trod if the Spirits don't appear to him.
Earlier in his visit, Marley offers his own life as a warning to Scrooge and as the reason for his haunting Scrooge at this moment, seven years after his own death, "seven years ago, this very night.”
“It is required of every man,” the Ghost [Marley] returned, “that the spirit within him should walk abroad among his fellowmen, and travel far and wide; and if that spirit goes not forth in life, it is condemned to do so after death. It is doomed to wander through the world—oh, woe is me!—and witness what it cannot share, but might have shared on earth, and turned to happiness!”
Marley also warns Scrooge that he's learned "that no space of regret can make amends for one life's opportunities misused!"
When Scrooge remarks that Marley was always "a good man of business," Marley cries out, "Mankind was my business." Marley says that the common welfare, charity, mercy, forbearance, and benevolence were also his business, another warning to Scrooge not to follow in Marley's path to unavailing grief and eternal regret, which troubles Marley most "at this time," meaning at Christmastime, the time of year when Scrooge is most uncharitable.
The final warning to Scrooge comes not from Marley himself, but from the many restless, lamenting, wailing phantoms whom Marley joins outside Scrooge's window when he leaves Scrooge's rooms.
The misery with them all was, clearly, that they sought to interfere, for good, in human matters, and had lost the power for ever.
When the phantoms fade into the mist and Scrooge is left alone, Scrooge starts to say "Humbug!" but he stops at the first syllable. Whether Scrooge realizes it or not, Marley's warnings are starting to have an effect on him. It's left to the Three Sprits to finish teaching Scrooge the lessons that Marley wants him to learn.