Context. Lyman-break galaxies (LBG) and narrowband (NB) surveys have been successful at detecting large samples of high-redshift galaxies. Both methods are subject to contamination from low-redshift interlopers. Aims. Our aim is to investigate the nature of low-redshift interlopers in NB Lyman-ff emitters (LAE) searches. Methods. From previous HAWK-I NB imaging at z similar to 7.7, we identify three objects that would have been selected as high-redshift LAEs had our optical data been one magnitude shallower (but still one to two magnitudes fainter than the near infrared data). We follow up these objects in spectroscopy with X-Shooter at the VLT. Results. Despite low quality data due to bad weather conditions, for each of the three objects we identify one, and only one emission line, in the spectra of the objects that we identify as the [OIII]5007 angstrom line. From this result, combined with spectral energy density fitting and tests based on line ratios of several populations of galaxies, we infer that the three objects are ultra-strong line emitters at redshifts similar to 1.1. Conclusions. From this work and the literature we remark that the [OIII] line appears to be a common source of contamination in high-redshift LBG and LAE samples and we suggest that efforts be made to characterize with high accuracy the [OIII] luminosity function out to redshift similar to 3 or higher.