The quantum adiabatic theorem states that a gapped system follows its instantaneous eigenstate during sufficiently slow evolution. We demonstrate that adiabaticity can fail even in arbitrarily slow, resonance-free processes. Introducing Instantaneous Transition Accumulation (ITA) and Instantaneous Transition Probability (ITP)—parameters capturing cross-Berry connections and eigenstate amplitudes—we reveal overlooked dynamic-geometric mechanisms driving transitions. Using a new Phase Difference Manipulation (PDM) method, we control ITP and ITA to induce adiabaticity violation in a Landau-Zener (LZ) process. We experimentally demonstrate this counterintuitive phenomenon in a photonic waveguide system, where a slow LZ process defies adiabaticity, switching energy levels despite a fivefold slower evolution speed than a conventional adiabatic process. This discovery reshapes our understanding of quantum evolution and holds potential for quantum computing, topological physics, and photonic technologies.