A peculiar galaxy appears at redshift 11: properties of a moderate-redshift interloper

In 2011 Laporte et al. reported a very high redshift galaxy candidate: a lensed J-band dropout (A2667-J1). J1 has a photometric redshift of z= 9.612, the probability density function for which permits no low- or intermediate-z solution. We here report new spectroscopic observations of this galaxy with Very Large Telescope/X-Shooter, which show clear [O iii] ?5007 angstrom, Lya, Ha and H beta emission and place the galaxy firmly at z= 2.082. The oxygen lines contribute only similar to 25 per cent to the H-band flux and do not significantly affect the dropout selection of J1. After correcting the broad-band fluxes for line emission, we identify two roughly equally plausible natures for A2667-J1: it is either a young heavily reddened starburst or a maximally old system with a very pronounced 4000-angstrom break, upon which a minor secondary burst of star formation is superimposed. Fits show that to make a 3s detection of this object in the B band (V band), imaging of depth AB = 30.2 (29.5) would be required despite the relatively bright near-infrared (NIR) magnitude, we would need optical data of equivalent depth to the Hubble Ultra Deep Field to rule out the mid-z solution on purely photometric grounds. Assuming that this stellar population can be scaled to the NIR magnitudes of recent Hubble Space Telescope/Wide Field Camera 3 (WFC3) IR-selected galaxies, we conclude that unfeasibly deep optical data (reaching AB similar to 32) would be required for the same level of security. There is a population of galaxies at z similar to 2 with continuum colours alone that mimic those of our z= 712 candidates.