I noticed that when I didn't spell anything wrong, I was told that there were no errors. When I did spell something wrong, however, I got a "server error". Looking at the RC logs, I saw that it was unable to find the file spell_googlie.inc.
After looking in more detail at the error message, and the directory listings, I ended up getting spelling to work just fine by renaming a file:
Old:
/program/steps/mail/spell_googie.inc
New:
/program/steps/mail/spell_googlie.inc
I don't know if this was some kind of strange typo / bug in the spelling of this file name or not. But renaming it to put the "l" in fixed everything.