  We report an exceptional mid-infrared flare in the Seyfert 1.8 NGC 3786. In
the multi-epoch data from the Wide-field Infrared Survey Explorer, the nuclear
mid-infrared brightness of NGC 3786 appears to vary substantially up to
$0.5-0.8$ mag around mid-2020. However, there is no evidence of significant
variation in the corresponding light curve of the optical band from the Zwicky
Transient Facility. This implies that the flare may have been heavily obscured
by nuclear dust. Through follow-up spectroscopic observations with Gemini-North
after the flare, we find that broad emission lines in ${\rm Pa}\alpha$ and
${\rm Pa}\beta$ newly appear, while the broad ${\rm H}\beta$ emission is
marginally detected in the post-flare spectrum. In addition, their central
wavelengths are systematically redshifted up to 900 km s$^{-1}$ with respect to
the narrow emission lines. This reveals that the flare is associated with the
changing-look phenomenon from type 1.8 to type 1. Based on these findings, we
argue that the flare is likely to originate from an obscured tidal disruption
event, although extreme variation in the accretion rate may not be ruled out
completely.
