  We investigate the possibility of contamination by lower-redshift interlopers
in the measure of the ionising radiation escaping from high redshift galaxies.
Taking advantage of the new ultra-deep VLT/VIMOS U-band number counts in the
GOODS-S field,we calculate the expected probability of contamination by low-z
interlopers as a function of the U-mag and the image spatial resolution (PSF).
Assuming that ground-based observations can not resolve objects lying within a
0.5" radius of each other, then each z>=3 galaxy has a 2.1 and 3.2% chance of
foreground contamination, adopting surface density U-band number counts down to
27.5 and 28.5,respectively. Those probabilities increase to 8.5 and 12.6%,
assuming 1.0" radius. If applied to the estimates reported in the literature at
z~3 for which a Lyman continuum has been observed directly,the probability that
at least 1/3 of them are affected by foreground contamination is larger than
50%.From a Monte-Carlo simulation we estimate the median integrated
contribution of foreground sources to the Lyman continuum flux (f900). Current
estimations from stacked data are >2 sigma of the median integrated pollution
by foreground sources.The spatial cross-correlation between the U-band catalog
and a sample of galaxies at z>=3.4 in the GOODS-S field,produces a number of
U-band detected systems fully consistent with the expected superposition
statistics. Indeed, each of them shows the presence of at least one offset
contaminant in the ACS images. An exemplary case of a foreground contamination
in the HUDF at redshift 3.797 by a foreground blue compact source (U=28.63 +/-
0.2) is reported; if observed with a seeing larger than 0.5" the polluting
source would mimic an observed (f1500/f900)_OBS ~ 38, erroneously ascribed to
the source at higher redshift. [Abridged]
