In a single slit diffraction pattern, identify the incorrect statement from the following.

Single-slit diffraction produces a pattern where light waves bend around obstacles or spread after passing through a narrow aperture. The central maximum has angular width $2\lambda/a$, while secondary maxima each have width $\lambda/a$, establishing unequal fringe widths.

Intensity distribution shows the central maximum as brightest, with secondary maxima intensities decreasing rapidly—the first secondary maximum is approximately $4.5\%$ of the central intensity. This confirms unequal intensities.

Option D claims fringes have equal width and equal intensity, which contradicts the fundamental properties of single-slit diffraction. This pattern distinctly features both unequal widths and unequal intensities.

The incorrect statement is $\boxed{D}$.