  Gravitational waves (GW) from chirping binary black holes (BBHs) provide
unique opportunities to test general relativity (GR) in the strong-field
regime. However, testing GR can be challenging when incomplete physical
modeling of the expected signal gives rise to systematic biases. In this study,
we investigate the potential influence of wave effects in gravitational lensing
(which we refer to as microlensing) on tests of GR using GWs for the first
time. We utilize an isolated point-lens model for microlensing with the lens
mass ranging from $10-10^5~$M$_\odot$ and base our conclusions on an
astrophysically motivated population of BBHs in the LIGO-Virgo detector
network. Our analysis centers on two theory-agnostic tests of gravity: the
inspiral-merger-ringdown consistency test (IMRCT) and the parameterized tests.
Our findings reveal two key insights: First, microlensing can significantly
bias GR tests, with a confidence level exceeding $5\sigma$. Notably,
substantial deviations from GR $(\sigma > 3)$ tend to align with a strong
preference for microlensing over an unlensed signal, underscoring the need for
microlensing analysis before claiming any erroneous GR deviations. Nonetheless,
we do encounter scenarios where deviations from GR remain significant ($1 <
\sigma < 3$), yet the Bayes factor lacks the strength to confidently assert
microlensing. Second, deviations from GR correlate with pronounced interference
effects, which appear when the GW frequency ($f_\mathrm{GW}$) aligns with the
inverse time delay between microlens-induced images ($t_\mathrm{d}$). These
false deviations peak in the wave-dominated region and fade where
$f_\mathrm{GW}\cdot t_\mathrm{d}$ significantly deviates from unity. Our
findings apply broadly to any microlensing scenario, extending beyond specific
models and parameter spaces, as we relate the observed biases to the
fundamental characteristics of lensing.
