On your Mac the stack trace hints that the bug is due to a bad pointer/offset/size being passed to fwrite() but in the debug builds it looks like it's in the fa

On your Mac the stack trace hints that the bug is due to a bad pointer/offset/size being passed to fwrite() but in the debug builds it looks like it's in the fastcmp() function. Is this the descrepancy you're talking about in the final paragraph?