  Although coronal mass ejections (CMEs) resembling flux ropes generally expand
self-similarly, deformations along their fronts have been reported in
observations and simulations. We present evidence of one CME becoming deformed
after a period of self-similarly expansion in the corona. The event was
observed by multiple white-light imagers on January 20-22, 2021. The change in
shape is evident in observations from the heliospheric imagers from the
Wide-Field Imager for Solar Probe Plus (WISPR), which observe this CME for
$\sim$ 44 hours. We reconstruct the CME using forward-fitting models. In the
first hours, observations are consistent with a self-similar expansion but
later on the front flattens forming a dimple. Our interpretation is that the
CME becomes deformed at $\sim0.1\ au$ due to differences in the background
solar wind speeds. The CME expands more at higher latitudes, where the
background solar wind is faster. We consider other possible causes for
deformations, such as loss of coherence and slow-mode shocks. The CME
deformation seems to cause a time-of-arrival error of 16 hours at $\sim 0.5\
au$. The deformation is clear only in the WISPR observations and, it thus,
would have been missed by 1~AU coronagraphs. Such deformations may help explain
the time-of-arrival errors in events where only coronagraph observations are
available.
