  Parameter estimates of GW150914 were obtained using Bayesian inference, based
on three semi-analytic waveform models for binary black hole coalescences.
These waveform models differ from each other in their treatment of black hole
spins, and all three models make some simplifying assumptions, notably to
neglect sub-dominant waveform harmonic modes and orbital eccentricity.
Furthermore, while the models are calibrated to agree with waveforms obtained
by full numerical solutions of Einstein's equations, any such calibration is
accurate only to some non-zero tolerance and is limited by the accuracy of the
underlying phenomenology, availability, quality, and parameter-space coverage
of numerical simulations. This paper complements the original analyses of
GW150914 with an investigation of the effects of possible systematic errors in
the waveform models on estimates of its source parameters. To test for
systematic errors we repeat the original Bayesian analyses on mock signals from
numerical simulations of a series of binary configurations with parameters
similar to those found for GW150914. Overall, we find no evidence for a
systematic bias relative to the statistical error of the original parameter
recovery of GW150914 due to modeling approximations or modeling inaccuracies.
However, parameter biases are found to occur for some configurations disfavored
by the data of GW150914: for binaries inclined edge-on to the detector over a
small range of choices of polarization angles, and also for eccentricities
greater than $\sim$0.05. For signals with higher signal-to-noise ratio than
GW150914, or in other regions of the binary parameter space (lower masses,
larger mass ratios, or higher spins), we expect that systematic errors in
current waveform models may impact gravitational-wave measurements, making more
accurate models desirable for future observations.
