A two-dimensional photonic crystal microcavity design supporting a wavelength-scale volume resonant mode with a calculated quality factor (Q) insensitive to deviations in the cavity geometry at the level of Q >= 2 x 10^4 is presented. The robustness of the cavity design is confirmed by optical fiber-based measurements of passive cavities fabricated in silicon. For microcavities operating in the λ = 1500 nm wavelength band, quality factors between 1.3-4.0 x 10^4 are measured for significant variations in cavity geometry and for resonant mode normalized frequencies shifted by as much as 10% of the nominal value.