Thanks Joe, I did end up figuring out what was causing it and you are exactly correct. I probably should have posted here that I had figured it out!
All the templates on this clients site were not properly implementing the main loop. The Yoast plugin, like you say, does in fact call the_post somewhere in it’s stack. Turning it off killed that call, causing the templates to break.
I would upvote your stackexchange question and answer, but of course I don’t have enough reputation. Great feature on those sites, lol.
