  Identifying sources exhibiting ellipsoidal variability in large photometric
surveys is becoming a promising method to search for candidate detached black
holes in binaries. This technique aims to exploit the orbital-phase dependent
modulation in optical photometry caused by the black hole distorting the shape
of the luminous star to constrain the mass ratio of the binary. Without
understanding if, or how much, contamination is present in the candidate black
hole samples produced by this new technique it is hard to leverage them for
black hole discovery. Here, we follow up one of the best candidates identified
from Gaia Data Release 3, Gaia DR3 4042390512917208960, with a radial velocity
campaign. Combined photometric and radial velocity modelling, along with
spectral disentangling, suggests that the true mass ratio (mass of the unseen
object divided by the mass of the luminous star) is an order of magnitude
smaller than that inferred assuming the modulations arise from ellipsoidal
variability. We therefore infer that this system is likely a contact binary, or
on the boundary of both stars nearly filling their Roche lobes, however,
further observations are required to confidently detect the secondary. We find
that the well-known problem of discriminating between ellipsoidal and contact
binary light curves results in a larger contamination from contact binaries
than previously suggested. Until ellipsoidal variables can be reliably
distinguished from contact binaries, samples of black hole candidates selected
based on ellipsoidal variability are likely to be highly contaminated by
contact binaries or similar systems.
