  We study how the non-adiabatic effect causes the observable fluctuation in
the "geometric phase" for a two-level system, which is defined as the
experimentally measurable quantity in the adiabatic limit. From the Rabi's
exact solution to this model, we give a reasonable explanation to the
experimental discovery of phase fluctuation in the superconducting circuit
system [P. J. Leek, \textit{et al}., Science \textbf{318}, 1889 (2007)], which
seemed to be regarded as the conventional experimental error.
