Star Wars has hinted at a secret plan cultivated by Palpatine during the Clone Wars. Darth Sidious was the very definition of an evil mastermind; his plans operated on many levels, and he was skilled at reacting to unforeseen events. The clone army - commissioned by Jedi Master Sifo-Dyas after he experienced visions of the conflicts to come - could have been his undoing, but instead Palpatine used them to engineer the Clone Wars.
Sometimes, of course, it was easy to see how events could be used to his benefit - especially with the rise of the dark side, which meant things just seemed to work out exactly how Palpatine would have wanted. A perfect example can be seen in Star Wars: The Clone Wars season 5, in an arc in which Ahsoka Tano was framed for a terrorist attack on the Jedi Temple. The actual perpetrator was her disillusioned fellow Padawan, Barriss Offee; although Barriss was exposed, the experience shook Ahsoka's faith in the wisdom of the Jedi Council, and Ahsoka chose to leave the Jedi. This helped further isolate Anakin Skywalker, leaving him as shaken as Ahsoka.
Lucasfilm's recently-published anthology, Stories of Jedi & Sith, contains a short story that hints that the Sith may have been more involved in these events than had seemed to be the case. A Jedi's Duty, by Karen Strong, is a short story told from the perspective of Barriss Offee before she began to fall to the dark side, and it reveals Barriss could sometimes sense a dark presence in the corridors of the Jedi Temple. "Darkness roamed its corridors and stalked her movements. Barriss had sensed its lurking existence ever since she returned." Palpatine did indeed use the dark side, even in the Temple, no doubt amused to hide it from the greatest Jedi Masters. It's easy to imagine him identifying weak and vulnerable Jedi like Barriss Offee, who could be affected by his darkness and potentially turned. While this doesn't suggest Palpatine engineered Barriss Offee's fall, it probably means he was there pushing her toward the cliff edge.
The Jedi are believed to have suffered a large number of defections during the Clone Wars, and they may well have been encouraged by Palpatine in a similar manner. Indeed, this may well explain how he successfully identified Jedi who could be seduced to the dark side and recruited to serve as Inquisitors after Order 66. Palpatine could have been working on some of these Jedi for years. There's even been speculation Barriss Offee herself became an Inquisitor, although if so, she's yet to make an appearance in canon content set during the Dark Times of the Empire's reign. Certainly, Palpatine never believed in wasting a resource, so he'd have been interested in finding a use for her rather than simply having her executed.
It's reasonable to assume Palpatine chose to exert his will upon Barriss Offee (and, presumably, others like her) at times when few Masters were in the Jedi Temple. As Chancellor, he was kept abreast of Jedi movements, so he'd have known how to pick his moments. Star Wars' greatest villain truly was a ruthless mastermind.
