ExpressJS res.render() error (JSON.stringify can't work on circular reference)
This was a problem with session data being modified in a view.
After much digging, I found that it was a bug in the way partials are handled in 2.5.8. I submitted an issue, and subsequently a patch. (in case anyone needs this info at a future date) as npm is still serving up Express 2.5.8 AFAIK.
Thanks for your help @freakish and @Ryan