Recently, A. S. Riggs of the United States Geological Survey has reported an instance where the assumption was not true.
The paper where Riggs reported this is from 1984 -
Science, vol 224, pp 58-61. It, in turn, refers to work as early as 1954 which points out this effect. I've seen Riggs' paper used as a footnote on some YEC site - the site author obviously didn't expect his readers to actually look the paper up or anything. Another footnote on the same site was to "Radiocarbon Dating: Fictitious Results with Modern Shells," M L Keith, G M Anderson,
Science, 141, 634-637 (1963): the footnote didn't give the title, though - merely quoted the shell's date without explaining that the paper explained why it was fictitious.
This "snail shell argument" is so very poor that even the Institute for Creation Research disavowed it, back in 1989!
The shells of live freshwater clams can, and often do, give anomalous radiocarbon results. However, the reason for this is understood and the problem is restricted to only a few special cases, of which freshwater clams are the best-known example. It is not correct to state or imply from this evidence that the radiocarbon dating technique is thus shown to be generally invalid.
(from
Acts and Facts Magazine | The Institute for Creation Research , question #3)