You can easily miss it with the wrong text font in NI. I'm using "Consolas" which shows placeholder symbols for unprintable characters on my system. I've seen only a single spell resource so far (#BONECIR.SPL) with an incorrect signature while browsing through the list.
Comments
Output from CLONE_EFFECT would have been to the effect of
WARNING: CLONE_EFFECT does not support file type SPL\unprintable_character