When Pippin recovers the palantír in “The Two Towers,” Gandalf quickly snatches it away, leaving the hobbit with tempting memories of the mesmerizing Seeing Stone. Eventually, that itch in Pippin’s palms becomes so strong that he sneaks the stone away from the Wizard, looks in it, and almost ruins the entire quest when he’s revealed to Sauron and nearly tells him everything.
In “The Two Towers” book, when Gandalf realizes what has happened, he admits, “[Pippin] ought never to have touched it in Isengard, and there I should have been quicker.” He offers the excuse of being distracted by Saruman and then, afterward, of being so tired that he fell asleep while thinking about what to do with it. It’s clear that the Wizard knows it’s a powerful, dangerous item, though, and that he should have taken better precautions.
This also alludes to an even more terrible “might have been.” After seeing what happens to Pippin, Gandalf confesses, “I had considered whether or not to probe this Stone myself to find its uses. Had I done so, I should have been revealed to [Sauron] myself.” After adding that he isn’t strong enough for a test like that, he concludes, “It would be disastrous for him to see me, yet.” The entire palantír episode comes close to ruining everything — and Gandalf’s poor decision-making is a catalyst for the entire affair.