A method was called at an unexpected time.. The exception would occur seemingly at random (sometimes it would happen, sometimes it wouldn't provided the exact same data which indicated I might have a race condition that's occurring). My data was being loaded via a string and not via any HTTP calls.
As an aside, I've also have had inconsistent luck with using
Eval which always work. I'm sure there's a gotcha I'm not aware of and if I find it I'll update this post.