We demonstrate that partial wave amplitudes extracted from $D^+\to K^-\pi^+\pi^+$ Dalitz plot with a unitary coupled-channel model are significantly different from those obtained with an isobar model. The unitary coupled-channel model takes account of hadronic rescattering mechanisms involving all three mesons that have been missed in conventioanl isobar model analyses. The rescattering mechanisms contribute largely, and can triplicate the $D^+\to {K}^-\pi^+\pi^+$ decay width within our analysis. These findings deliver a warning that analysis results obtained with isobar models should be looked with a caution. The determination of the CKM angle $\gamma/\phi_3$ is a highly relevant problem.