  Next-generation terrestrial gravitational-wave observatories will detect
$\mathcal{O}(10^{5})$ signals from compact binary coalescences every year.
These signals can last for several hours in the detectors' sensitivity band and
they will be affected by multiple unresolved sources contributing to a
confusion-noise background in the data. Using an information-matrix formalism,
we estimate the impact of the confusion noise power spectral density in
broadening the parameter estimates of a GW170817-like event. If our estimate of
the confusion noise power spectral density is neglected, we find that masses,
spins, and distance are biased in about half of our simulations under realistic
circumstances. The sky localization, while still precise, can be biased in up
to $80\%$ of our simulations, potentially posing a problem in follow-up
searches for electromagnetic counterparts.
