I get the interesting error below when I try to compile features in an OT script font. The notable things are that the claimed number of elements is absurdly large, and the error message is claiming that a number is not equal to itself. The only features in the font are calt and maybe kern.

It seems to me that I have seen this error before, and it does mean something, but as the error message itslf is bogus, I can't recall for sure what that is. Is it related to overflow of the 64K size limit on a subtable in OpenType? That rings a bell for me....