  We report an unexpected variation in the positional offset distributions
between Herschel-ATLAS sub-millimetre (submm) sources and their optical
associations, depending on both 250-{\mu}m signal-to-noise ratio and
250/350-{\mu}m colour. We show that redder and brighter submm sources have
optical associations with a broader distribution of positional offsets than
would be expected if these offsets were due to random positional errors in the
source extraction. The observation can be explained by two possible effects:
either red submm sources trace a more clustered population than blue ones, and
their positional errors are increased by confusion; or red submm sources are
generally at high redshifts and are frequently associated with low-redshift
lensing structures which are identified as false counterparts. We perform
various analyses of the data, including the multiplicity of optical
associations, the redshift and magnitude distributions in H-ATLAS in comparison
to HerMES, and simulations of weak lensing, and we conclude that the effects
are most likely to be explained by widespread weak lensing of Herschel-SPIRE
sources by foreground structures. This has important consequences for
counterpart identification and derived redshift distributions and luminosity
functions of submm surveys.
