I can't find this documented anywhere, but I am thinking there is a maximum number associated with the number of matches that can be counted with the 'body-contains' rule, for example;
body-contains("someword",1000) - works just fine
...but using say a count of 10,000 (a lot, I know!) does not.
[EDIT : I figured this out, 10,000 works fine! Apologies for the hasty post]