Two papers which examine this are:

M. L. Keith & G. M. Anderson, "Radiocarbon dating: Ficticious results
      with mollusc shells", Science, v141, 16 Aug 1963, pp634-637
M. Rubin & D. W. Taylor, "Radiocarbon activity from living clams and snails",
      ibid., p637

Both papers examine the mechanisms by which sources of old C14 (especially
carbonate ion in water from C14-inactive limestone, but also carbon from
organic detritus) can enter the shells of these, and thus provide an
explanation for the error, not merely reporting the fact of the error.

The original hoaxer in this thread needs to be a bit more careful about his
quoting, though. An error of 4000 years was claimed, but the mapers mention
errors of a mere 3000 :-)

The simple resolution is that carbon in shells, particularly those which
grew in a lime-rich environment, cannot be used for C14 dating. Similarly,
care must be taken to avoid any other organisms which may be recieving
"old" carbon from some source.