  The remarkable stability of extragalactic jets is surprising, given the
reasonable possibility of the growth of instabilities. In addition, much work
in the literature has invoked this possibility in order to explain observed jet
structures and obtain information from these structures. For example, it was
recently shown that the observed helical structures in the jet in S5 0836+710
could be associated with helical pressure waves generated by Kelvin-Helmholtz
instability. Our aim is to resolve the arc-second structure of the jet in the
quasar S5 0836+710 and confirm the lack of a hot-spot (reverse jet-shock) found
by present observing arrays, as this lack implies a loss of jet collimation
before interaction with the intergalactic medium. In this work, we use an
observation performed in 2008 using EVN and MERLIN. The combined data reduction
has provided a complete image of the object at arc-second scales. The lack of a
hot-spot in the arc-second radio structure is taken as evidence that the jet
losses its collimation between the VLBI region and the region of interaction
with the ambient medium. This result, together with the previous identification
of the helical structures in the jet with helical pressure waves that grow in
amplitude with distance, allow us to conclude that the jet is probably
disrupted by the growth of Kelvin-Helmholtz instability. This observational
evidence confirms that the physical parameters of jets can be extracted using
the assumption that instability is present in jets and can be the reason for
many observed structures. Interestingly, the observed jet is classified as a
FRII object in terms of its luminosity, but its large-scale morphology does not
correspond to this classification. The implications of this fact are discussed.
