Well, THAT was frustrating!
Originally Posted by mncowboy
... and make sure the guide item truly matches this in your opf file.
It did match... however, this comment lead me to the issue. In my .opf file, my guide pointed to filename.html%23
toc instead of filename.html#
toc. I have no idea how it got the code for the # symbol, though I'm guessing user error.
Thank you all for the replies. Problem resolved.