  We present deep WFC3 grism observations of the candidate z~12 galaxy
UDFj-39546284 in the HST Ultra Deep Field (UDF), by combining spectroscopic
data from the 3D-HST and CANDELS surveys. The total exposure time is 40.5 ks
and the spectrum covers 1.10 < {\lambda} < 1.65{\mu}m. We search for faint
emission lines by cross-correlating the 2D G141 spectrum with the observed H160
morphology, a technique that is unique to slitless spectroscopy at HST
resolution. We find a 2.7{\sigma} detection of an emission line at 1.599
{\mu}m---just redward of the JH140 filter---with flux 3.5 +/- 1.3x10-18
erg/s/cm2. Assuming the line is real, it contributes 110 +/- 40% of the
observed H160 flux and has an observed equivalent width > 7300 {\AA}. If the
line is confirmed, it could be Ly-{\alpha} at z = 12.12. However, a more
plausible interpretation, given current results, could be a lower redshift
feature such as [O III]{\lambda}4959,5007 at z = 2.19. We find two other 3D-HST
[O III] emitters within 1000 km/s of that redshift in the GOODS-South field.
Additional support for this interpretation comes from the discovery of a bright
"[O III] blob" with a secure G141 grism redshift of z = 1.605. This object has
a strikingly large observed equivalent width of nearly 9000 {\AA} that results
in similar "dropout" colors as UDFj-39546284.
