For my large project, I usually create a smaller project to test new loops and proceedures.
What I did is modify a core proceedure by adding a new tag, but I forgot to make those changes in the big project. So the new call proceedure was actually needing 4 arguments, and the root I had in my project was 3.
This did not show up as a warning. It is my own fault, but I got to the bottom of it.