The isobar model is an essential part of partial wave analyses
(PWA) of multi-body hadronic final states.
In the isobar model, all
final states are described as sequential two-body decays of
intermediate resonances.
It turned out to work surprisingly well in the past, despite the fact that
it neglects rescattering, i.e. interactions in the final state.
With the advent of data samples of extremely high statistical
significance from ongoing experiments, effects due to final state
interactions may become important in order to understand the data.
We discuss the example of the recently observed $a_1(1420)$ in the $3\pi$
final state using PWA techniques. In our model,
we interpret the signal as a result of rescattering in the final state.
In the presence of coupled $\pi\pi\pi$ / $K\bar{K}\pi$ channels,
final-state interaction causes a redistribution of
events between the $\pi\pi\pi$ and $K\bar{K}\pi$ systems.
We compare our interpretation to
the genuine resonance hypothesis and discuss a sophisticated approach to clarify the nature
of the signal.