Parametric down-conversion of x rays into extreme ultraviolet and accompanying nonlinear x-ray diffraction were investigated both theoretically and experimentally. The theory predicted a rocking curve having a Lorentzian signal peak; however, the experiments revealed a peak followed by an unexpected dip. Together with the scattering angle dependence of the signal, the experimental results implied a possibility of interference between the Compton scattering and the parametric down-conversion. The similarity to the Fano effect is discussed.