A "slip" in this sense could be a mistake. A "slip of the tongue" means to misspeak.
One of my mother's favorite proverbs is "there's many a slip twixt the cup and the lip," which means that plans don't always go the way one intends.
However, "slip" in your sentence could also mean the act of slipping, a smooth and easy movement. "With a slip of the finger, she finished tying the elaborate bow in Flora's hair." Or even a secretive movement: "With a slip of the finger, he whisked the pearl necklace into his pocket."
Can you give more context?
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